Yes. Lots of times. I recall one ride vividly where a friend led me out on a longer ride than I had counted on. It was about 65 miles out and back and fairly hilly. We were most of the way home, about 12 miles to go, on an uphill, and my legs Just. Wouldn't. Turn. Had to get off and walk. That particular hill was one I'd ridden plenty of times, but that time, I didn't have enough left to climb it.