My worst marathon ever - I rate it as worse than my DNF - in about 3 dozen events came about a week after giving blood. At the time, I was conditioned to respond to the telephone calls and by the time I realized that I shouldn't be donating, I was too embarassed to say stop. Then I was too foolish not to run.
At least in Canada, the standard is that one must wait at least 56 days between donations. It takes that long for many people to regenerate fully.
Donating now will compromise your next few weeks of training and likely the marathon itself. Why not postpone the donation until a couple of weeks after? At that stage, you should be running modest distances only in the recovery phase and should have a reason to celebrate. Donating is doing something for the community and is very, very worthwhile, but it also can be done every two or three months for the next few decades.
If you are determined to donate and live in a larger center, you might want to explore alternatives. Apheris is a technique to extract platelets only from the donor's blood. The plasma and blood cells are re-injected into the donor. It takes a while to donate - likely a couple of hours - but does not appear to compromise training.
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