I donate regularly, and while I avoid riding that day, I ride the next and am fine. I think how you feel afterwards is probably mostly individual. But, that said, I always feel better if I drink lots of water before and after donating, and also take an extra multi-vitamin (with iron) the day before.

That and I treat myself to a volunteer cookie at the donation center. /nod It's all about the cookie!

Congrats on donating. I'm sure we all know someone who's needed blood or blood products, and it's important that we help if we can.