As a Running Coach most often I'll get clients that want to donate blood. What an Amazing Gift to be able to give ♥ The American Red Cross suggests that you don't resume extreme physical activity for about 24 hrs after the donation.
I do however have further restriction as a coach and as an athlete myself. I deeply value the affect the donation has on the athlete's body. Within 24hrs the missing plasma in the blood is replaced. However it takes 4-6 weeks for the red blood cells to be replenished in the body. The red blood cells is exactly what sends oxygen throughout out your body. It's very normal that you will expect to feel a little more fatigued than normal while you run and train. I would highly suggest if you are at all concerned about your training and the timing to your donation that you send me a message. My Rule is that if you are in Marathon training that I strongly urge you to wait till you have completed your race. If you are insistent on doing it during a long 18 week training schedule that you avoid at all costs doing in the last 4-6 weeks before the big day. This way your red blood cells have had a chance to refresh and you can feel your ultimate best. Further you want to make sure you drink lots of fluids and eat lots of iron rich foods to aide you along in your athletic recovery.
I make a difference and so Can You!