During exercise I am like two different people. One is telling me to "keep on" and the other is telling me to "stop".
Today, the Stop person won at a mile and half on the stationary bicycle/rowing machine. Yesterday I had done two miles, as I had the day before that. Today I quit after 1.5 miles, but I HAD done a couple of speed sprints... getting up to fifty miles per hour at one brief point.