If you're in average shape (e.g., you can climb several flights of stairs without needing long breaks on your way up), then you can hike 15 miles a day in Colorado in early August.
The #1 secret to doing more miles than you think you can do is getting an early start!
Wake up at 5 a.m., eat something, and start hiking! Enjoy the sunrise! Before you know it, you'll have 5-10 miles in before noon! If thunderstorms come in the afternoon, you can wait them out and not feel any pressure since you'll have most of your 15 miles done.
The #2 secret is learning to love going uphill and taking them steadily and as fast as comfortable. Don't go so fast that you need to take a break. Find an uphill pace that you can sustain for 30 minutes or more without a break. Just poke along steadily and vigorously. It's more important to move steadily than to sprint and stop.
Most healthy people could have walked next to me on the CDT. I was never walking fast. A 63-year old man hiked with me for half a day on the CDT.
Do the math: The reason I could do 35+ miles was I woke up early (4-5am) and finished late (9-10pm). Doing 15 hours of hiking at 3 miles an hour, gives you 45 miles. Doing 2 miles an hour (very modest pace), gives you 30 miles. So I walked just a bit faster than 2 miles an hour and found doing at least 35 miles a day sustainable. I'm not superman and I'm convinced that most people, with a bit of training can do what I did (if they wanted to).
Doing 15 miles/day in Colorado is a lot of work, but if you wake up early and tackle those uphills with a steady pace (even just 1 mph), then you'll achieve your goal!