First, well it was just one step at a time, honest 
Just good endurance I guess. Would be on the trail by 7AM (it was not light out until 6:30) and stop at 7PM. I also live in Boulder, CO and often visit altitude on weekends .As I mentioned in an earlier thread many hikes just in-town are around 7-8k. I will often be at the 10k-13k range when I play on weekends. That is the elevation the CT/CDT in Colorado tends to be on. SHRUG. Who knows…just had three weeks to do the trail so it is the pace I had to do.
As for a May thru-hike. Aye Carumba! Maybe in the lower elevations near Denver you can hike a little, but consider this fact: My friends hiked the Lost Creek Wilderness in early-mid June. This area, on the CT, is one of the first areas to open up in the high country in Colorado. There was still lots of snow! And that area is not far from the start of the trail. Mid-late June is the earliest you probably want to hike the CT.
Also, check out these links…notice th dates on the photos.
Deer Mtn (10k feet), April 24th:
http://gallery.backcountry.net/co04?&page=2
June 5th, Rocky Mtn NP, at, again 10k feet. Still very snowy!:
http://gallery.backcountry.net/co04/acc
Two weeks later I led a backpacking trip to 12200’. The snow rapidly melted:
http://gallery.backcountry.net/co04?&page=10
Typical Colorado snow conditions in early spring.
Just my .02 worth. Best of luck in whatever choice you make!
Mags