Can't help you with the guide question, but some hunting advice I can offer up.
First, start getting in shape NOW! Walk, run, bike, lift weights, mix it up, get your heart rate up, start building endurance and strength.
Second, get a pair of quality, GOOD FITTING boots NOW and get them broken in. Nothing will ruin a hunt faster than poor fitting boots and blisters.
Third, once you settle on a guide, call them and get info on where you will be hunting, then learn as much as you can about the area. Learn as much as you can about elk as well. The more you know and the harder you work, the harder the guide will work for you. Listen you your guide!
Fourth, make sure your bow, arrow, and broadheads are squared away. On a guided hunt (or any other really), equipment failure is not an option.
Lastly, get to Colorado 3 or 4 days before your hunt, go for some walks and short runs. Why? To help you acclimate to the altitude. Drink plenty of water and avoid alcohol, dehydration is the number one culprit of altitude sickness.
Do some search's here on TG, there is lots of great practical elk hunting and fitness advice from people that know way more than me. Hope this was of some help. Good luck! Hope to see some success pics in the fall!