Can't answer your question I'm afraid Saran, but thought I'd respond to the initial point.
Last time I gave blood, in the summer, being pig-headed, tall and heavy (48 yrs, 6ft 5, 14 stone), I went out and did a 16 miler the following evening. For the last half of the run I felt dreadful, getting so tired, and it was all I could do to finish.
3 weeks ago I did the Dublin marathon, and in 2 weeks' time I'm doing the Lisbon marathon (I've done 2 marathons in 8 days a couple of times in the past so I know my body can cope with this).
Inevitably (sod's law) I was sent an appointment to give blood last Thursday, 18 days before the second marathon. Really wasn't sure what to do and received wisdom here was inconclusive.
So I rang the blood people up and explained my dilemma. They advised caution. Don't give blood 2/3 weeks before my marathon, and then after the event leave it at least a week before making a donation, assuming that I seem to be recovering normally.
Fortunately I live in Bristol so it's possible to give a donation almost any day of the week, and I'll be able to do so before Christmas.
Hope this is of use to anybody who finds themselves with a similar dilemma.