Next month I'll be 82 and have been riding mostly one speeds for 30 years.
I'm a better runner than cyclist. I did finish 5th in vets class cyclocross nationals once, but there was snow, and I got to run up the hills.
The bike has saved my chronic achilles, and I consider it cross training, but in truth, I love it more than running.
Tried fixed gear, but scared myself on downhills. I don't know how you guys do it.