On the rare occasions when I do road running races, I can entertain myself for an embarrassingly long time doing slow math on reduced brain oxygen. "OK, that was 3.5 km in 17:26. So if I can keep it up, I'll be done in... Oh look, there's the 4 km marker. Better look at my watch and do some more arithmetic..."
I definitely do that on rides that I'm trying to accomplish something on. Apparently I can do math, but not count. Good thing I carry a GPS to do the counting.