I think your idea to do a loop is a good one. I've traveled and skied a lot in those states, but done no cycling, so can't offer specific advice about the riding spots.
Even with good conditioning, you'll likely find the altitude saps your endurance at first. Take time to acclimate and stay hydrated. The air is dry as well as thin. I did a ski trip to Keystone about 10 years ago. The house we rented was at 10,000 feet elevation, and I'd wake up in the middle of the night panting. Denver is just under a mile high. You might consider doing your loop to hit lower elevation spots first, finishing up in the high country.
It may be too hot to hit some of the more desert climate low elevation spots. On the other hand, the end of September will definitely be fall in the high country.
Have fun.
Even with good conditioning, you'll likely find the altitude saps your endurance at first. Take time to acclimate and stay hydrated. The air is dry as well as thin. I did a ski trip to Keystone about 10 years ago. The house we rented was at 10,000 feet elevation, and I'd wake up in the middle of the night panting. Denver is just under a mile high. You might consider doing your loop to hit lower elevation spots first, finishing up in the high country.
It may be too hot to hit some of the more desert climate low elevation spots. On the other hand, the end of September will definitely be fall in the high country.
Have fun.