One thing you want to do is get a copy of the big game regulations for CO. Study them very carefully, as this state has lots of little details that can trip you up. I think you can find them on the state wildlife website listed above.
For instance, evidence of sex must be still attached to the carcass in CO. Just boning out meat won't legally do it in this state. So be prepared to hump out one large part with the scrotum attached, at the least (antlers/head don't count, believe it or not).
Do the research on this sort of thing, it could save you some really big trouble.
Most of the other advice is excellent. I would redouble the part about getting into great physical shape. The problem with bow season is that it's early and therefore generally warm, and the elk will be high (maybe 10,000 - 11,000 feet up). Up around timberline where the trees stop. Simply walking on level ground at this altitude can make even a fit young person gasp for air. If you're climbing with a pack, it's step-step-stop-gasp-gasp-gasp-gasp and repeat ad nauseum. Not only do really heavy aerobic exercise, work your legs hard in the gym with weights for strength increase. Leg presses, parallel squats, that kind of thing. Between climbing with your packs and trying to control yourself carrying down heavy loads of elk meat in steep, rocky, rugged country, your leg muscles, knees, and ankles will take a severe beating.
Order through the U.S. Dept. of Interior, Bureau of Land Management (BLM), the Colorado state map showing land ownership in different colors. This will show you where the accessible public lands are. You will be interested in the National Forest (green areas) or BLM (yellow areas). Please note that any state lands (blue areas) that are not actually either state wildlife (hunting) areas or state trust hunting land (open to the public for hunting) are under the direct control of whoever is leasing it (local ranchers) and cannot be accessed without their permission. You can then get either 'section' BLM maps of the specific area you want to hunt, or 7.5 minute U.S. Geological Survey 'quad' maps. The state map is great to ponder at length (I have one up on my bedroom wall).
I hear great things about both the Craig and Gunnison areas for hunting success, but while I have driven through them I have never hunted them. There is mostly private land around Craig, with some national forest (Routt Nat'l Forest, Elkhead Mountains) about 25 miles to the northeast of it and lots of patchy checkerboard BLM about 30 miles northwest of Craig that trends into solid BLM. Not sure what the land there looks like, but the Elkheads are lovely. About 30 miles south of Craig is the Flattops Wilderness area, a very high grassy flat plateau with steep thickly wooded sides. The land down low around these areas will be totally open, and up high, thick pine and aspen groves.
Gunnison area will be pretty thick and rugged to the north towards Crested Butte, with lots of hunters. Northeast of Gunnison is the West Elk wilderness. Never been there (seen it from way off to the south), but it is high steep rugged country that should match your masochistic desire to foot-pack into it. To the south of Gunnison across Rt. 50, open bare grass and sage flats and hills (BLM land) trending uphill eventually to thicker brushier hills and eventually full forest on high national forest land.
You have to pick an area, find out what game unit it is, and figure out if that area has over-the-counter bull tags. Some areas are draw only, and most cow tags will be draw. The area west of the road between Rifle and Meeker in the northwest part of the state (north of I-70, west of Glenwood Springs and well north of Grand Junction) may in some units have excess cow tags that can be had without a lottery draw. Not sure what units they are, look on the state website noted above. This is mostly BLM open country. East of that is the Flattops area, much higher thicker forest.
Another area worth looking at is in south-central CO along the continental divide southeast of Gunnison. Find the highway 114 which departs southwards of Rt. 50 a few miles east of Gunnison. It runs south through BLM, then turns southeast and rises into national forest over the continental divide at North Pass. Then it descends along a long valley into the tiny village of Sagauche (pronounced Sawatch). On either side of North Pass and across it on the divide, there's plenty of high forest with lots of little dirt access roads, especially on the east side of the pass. You can drive in as far as you can and start hoofing it. If you go southwest along the divide, you can head to the La Garita wilderness. If you go northeast along the divide, you'll head towards Sargents Mesa and Marshall Pass. Either way the scenery is stunning. If you drop down lower than the divide crest, you get into more mixed open/forested country, trending down to more desert-like areas at the valley bottom. Better for rifle than bow. Get the state map and peruse it, then try to get out here for a few days of scouting whatever location you pick.
The Pagosa Springs area is beautiful, but after hunting there the last nine seasons, I can't wait to try somewhere else. Too thick and too few critters, I've seen only one buck in nine years (got him) and two shootable bulls neither of which I could even shoulder the rifle on. Most years we don't see but a couple of does and a cow or two between 4-5 guys in camp. We're going to have a pow-wow shortly to determine where to go next, and I'm going to push for the Saguache/North Pass area.
Figure your packs will weigh 65-75 pounds, get really good ones like mentioned previously. MRE's are not light, you might want to carry plenty of pasta (ramen noodles) and freeze-dried stuff, oatmeal, etc. The water purifier pump is absolutely necessary, but be aware that there is actually very little water available in the state, so you'll want to find a creek/stream on the way in and follow it upwards and camp not too far from it. In fact make that part of your hike-in plan if you can find one on a map. Get a couple of collapsible soft plastic water containers that can hang from a tree with a spigot, and take care they don't freeze on you (at night pull them down and store them spigot-upwards after draining out the spigot tube, so if it freezes it won't split the plastic).
Some kind of camp stove will be necessary since fires are frequently banned as noted earlier. A small but powerful backpacking stove using liquid white gas should do the trick. Figure at least a gallon of fuel for two guys for a week (propane stoves mean carrying the steel propane bottles; too heavy..other fuels mostly do not possess the BTU's per weight of Coleman fuel, and may be more expensive). Better to carry fuel out than run out while still in the woods. I see that you can get white gas in a half-gallon can too in some places these days, one gallon can and one half-gallon can should be more than enough. As a traveling companion of mine once said while vacationing with me out here in a cold October, "if you have a gallon of Coleman fuel, you're not going to freeze to death".
As mentioned previously, weather can change in an hour or two from warm/sunny (the sun is VERY hot here and can burn you to a crisp) to cloudy/windy/cold/rain/snow. Experiencing three different seasons in a day is not unusual here, especially in the mountains - they make their own weather, and delight in trying to kill you with it.
Since being caught out in a sudden rain/snow storm is a likelihood, something to keep in mind is that wearing cotton can kill you. Your sweat (or rain) soaks it and when wet, it not only loses almost all its insulating qualities, it will suck the heat out of you faster than you will believe. Invest in synthetic 'wicking' long underwear sets of the light to medium weight thickness (Patagonia Capilene, REI, Eastern Mountain Sports Bergeline, or old-fashioned polypropylene). Scrounge up a wool shirt and a wool sweater or pile/fleece pullover. A fleece, down or synthetic-fill vest is useful too. On warm days you can wear cotton but if the weather starts to turn bad get the hell back to camp RIGHT EFFING NOW! unless you are carrying a full change of warm/damp-resistant clothes or storm clothes in your day pack and can change quickly. You can get T-shirts of this synthetic material too, I highly reccomend them.
I usually wear light synthetic long johns under cotton pants in cool weather, but wear either wool or synthetic (Micro-tech) pants in the cold. Upper layer is a zipper-vent turtleneck of lightweight synthetic in cool to warmer weather, or if it's really warm, the synthetic T-shirt. The main thing is that the layer next to your skin stay dry (and therefore warm), which is what the wicking synthetic accomplishes. The next layer out from that should be something that can absorb the sweat moisture wicked out from the underwear and still stay warm - hence the wool shirt and fleece, which works great. If you wear any down filled stuff, don't ever let it get wet.
Bowhunting you will hit chilly mornings, probably mostly warmish to hot days, and chilly late afternoons/evenings. Be aware that when the sun gets low it can get downright COLD, RIGHT NOW. That also goes for when the clouds roll in and the wind picks up.
The sun makes it feel about 10-15 degrees warmer than it actually is, and can fool you badly. Say it's 45 degrees, bright sun, little wind, feels like 55-60, you're stripped down to a T-shirt or something, then around 3 PM the clouds roll in, wind kicks up, no sun (or it's late enough to be low), all of a sudden it's 35 degrees with a wind chill factor pushing 15-20, and hypothermia is right around the corner, especially if you're wearing a cotton T-shirt and cotton flannel shirt all damp with sweat from climbing a hill in the sun. Get proper clothes and WATCH THE WEATHER!!
I highly reccomend hunting together as a pair and not splitting up. If you are in rugged, rumply, thick country you can get lost easier than you can imagine (voice of experience here!). Since you'll be hunting strange country after packing in a ways, there'll be no roads, power lines, etc. to help you find your way back to camp or car. Have good maps, a compass (know how to use them!) and a GPS if you can beg, borrow, or steal one. Program in the location of your car, and the location of your camp. When you finally pick the spot you want to go, try to find not only the best access road to drive in and park, but try to pick an unmistakeable route to hike in by and hike back out by, so you can easily find your car again - say along a particular ridge or valley. Be very very careful about your navigation; don't assume that if you just go wandering into the wilderness, that you'll be able to find your way right back to the car easily! Unless you're using marked foot trails (something to NOT count on!) you'll have to really watch your land navigation. I could tell you some interesting stories about some misadventures I've had while going in no more than a mile or so.
Boy I wrote a lot. I well remember my first two-week drive-out vacation to CO, back in July '79 with my best friend. We hunted 'yotes for a week then went backpacking in the mountains in a couple of different places. Didn't know what we were doing at all, had marginal to terrible clothes and equipment, got caught by hailstorms in the mountains, hiked up in cotton T-shirts on sunny days in July and froze anyway, made a snowman out of the 8-foot high snowdrifts behind our timberline camp at about 10,500 feet (remember this is mid-July!) but overall had a wonderful time. Heck, I wish I was going along with ya, except for all that aerobic preparation...heheheh.
PREPARE CAREFULLY, ACT WISELY, AND HAVE A BLAST!!!
Oh by the way, if you've never been out here before, you'll be hooked and ruined for life...