I donate every 56 days....I always assume that the next 2 days are slower days (maybe 1-2 mph slower....19-20 mph vs 22-23 mph cruising).....I usually try to donate in the morning so have had several hours before my 12.4 mi commute in the evening.....I do drink a lot more water on those days......am 6' 1"203 lbs and am 61.......no problems doing this over the last few years...the blood bank nurses do not agree with what I am doing, and give me "the speech" every time, but again I am having no issues. By the third day, I go back to pushing myself.
Just continue to be smart......
If you are on your bike, you already are smart!