+1. The last time I donated, the phlebotomist was a bit worried that I would be riding back across town after donating - they recommend no "heavy exertion" for 24-48 hours afterwards. But after checking my resting pulse and BP, she decided I'd probably survive.
The last time I donated blood, I hopped on my bike immediately afterwards and almost passed out during my commute. I felt fine when I left, but as soon as I was chugging up a hill, the darkness began to creep in around my eyes. I knew I was going down, so I stopped and drank some juice I had with me. I could hear my lungs and other vital organs screaming, "We need that blood back!" I barely made it home that evening. Whenever I sign up to donate now, I make sure that I don't bike to work that day!