Awesome, John! Whatever you end up doing will be an amazing adventure. Some things to think about from someone who grew up in Ft. Collins, spend a fair bit of time in Boulder, has lived in Denver, Colorado Springs, and now a wee mountain town a bit to the west:
-- Why not go to a mountain town instead of Ft. Collins or Boulder? They are not in the mountains, and so if you are relocating, you might as well go into the mountains. Grin. Doing this would mean you could bike out your front door and try various routes, at least in many places. It would put you at a higher altitude, however, so you'd need to allow for that.
-- Mountain towns easily accessible and within 90 minutes of Denver (at least they were in the early 90's when I was in college): Breckenridge, Fairplay, Cripple Creek/Victor (2 hours away?) Estes Park, and Grand Lake (several other towns in the vicinity as well), each with great Forest Service roads and excellent biking opportunities (check with the local bike store for route ideas. I'm less sure of Estes and Grand Lake and their biking opportunities, because much of their surrounding land in Rocky Mountain National Park, so investigate that if you are going there). Breckenridge and Fairplay both have various dirst roads up mountain passes that are amazing to bike and Boreas Pass out of Breckenridge is old railroad grade so 3-4% grade max I think (others are too, as narrow gauge railroads became dirt roads in many cases. I just don't remember the origins of Mosquito Pass or others. Out of Cripple Creek, loads of old railroad options: Old Stage Coach road, Phantom Cañon Road, Shelf Road (not sure if it was a railroad).
-- There is amazing family friendly riding connecting the towns along the tributaries of the N. Platte around Deckers/Buffalo Creek and the other wee towns there, and that is just S.W. of Denver (285 West to Pine, then South, to find it on the map). It's on various paved and dirt roads, wide, with great river (creek by most standards) access (fishing if you want/get a license). Kids love stops at the river. The Colorado Trail goes through here for a good hiking option, and Lost Creek Wilderness is very close as well.
Hope that helps. Ask any clarifying questions you have!
With abandon,
Patrick