A word of warning...
A few years ago, I was going to run/hike the Pawnee/Buchanan loop the first weekend of October. The Nedarland (8000 feet) weather forecast called for sun and 50*. It was 25* and snowing at Brainard Lake (10,500 feet). My dog loves snow so we did a short hike (2 miles out) before the conditions overwhelmed my shorts and fleece hoodie.
Fall in Colorado is tough for trip planning. There is a good chance the conditions will be perfect for running/hiking during the day or it could be the one weekend that winter shows up.
For those good conditions, I would plan on mid-50's during the day and high teen's for the nights. It will be windy on the passes/summits so bring a wind shirt. Chance of rain is low, snow would be more likely. The wind shirt should be able to handle light snow flurries. I would want a puffy jacket and long wool tights for the evening/morning. Basically, a shoulder season gear list.
If you are traveling to Colorado, I would recommend having a fallback plan (maybe Lost Creek Wilderness) if the forecast is crap.