How many MPD can you comfortably hike? Also, does this 5-6 day time frame include an acclimation period (strongly suggested BTW!)
As for mid-September, it can be the best time of the year to hike IMO.
Cool crisp nights, gorgeous days, less people, no insects, no t-storms.
Finally, public transit can serve quite well for backpacking in the summit county area
(Airport Bus to Denver, Greyhound to Frisco. Free summit county bus to various areas such as Copper, Frisco, Silverthorne,Breck and for $5 Leadville CO)