It's funny, but something as simple as a rest break hadn't occurred to me. D'oh!

So I took everyone's advice and did a somewhat extended version of my regular Sunday ride. I ended up going 13.5 miles, and felt great the whole time, even while tackling some killer hills. I think I've broken through my mental barrier!

BTW for the first time I wore cycling shoes (Exustars that I bought from a fellow TE-er). I don't have cleats, but I used them with my toe clips. What a difference the shoes made -- my pedaling felt much more efficient. I'm sure they contributed to my not being very tired during my ride.