Giving blood that close to a marathon is not a good idea. The main reason is it will reduce the oxygen carrying capacity of the blood. The fluid volume is replaced in a couple days, red blood cells in 4 or 5 weeks, and the iron/hemoglobin levels take up to 8 weeks to recover.
Wow, good to know, this explains a lot as to why I felt so fatigued I think? Would I feel fatigued because of it. Reason I asked is I felt physically strong all over but fatigued which I never felt during training. This in addition to all of the extra side to side movement I was doing in the first 10 miles navigating around runners/walkers.