Actually, back when I was in my 30s, I did a lot of double centuries (200 miles) with only one 10 minute stop every 50 miles in about 10 hours total time. Granted, I was riding most of the time drafting in a paceline, so it really wasn't all that hard. The main problem was a sore butt, and I'd be too fatigued to ride the next day. Now, I'm pretty much limited to metric centuries, although I'm riding them solo.