Not as much trouble as I’d expected from making a double red-cell donation as far as aerobic performance, but more than I’d expected as far as endurance: Quite tired after just 3 miles or so. Still a good run.
I’ve been giving blood several times a year for a while now. Maybe it’ll reduce my risk of infection, cancer, or Alzheimer’s as discussed in Iron Is the New Cholesterol. Worst case, it’s a very low risk way to help other people.