Just for completeness .... did my Half on Sunday after giving blood on Thursday and running a 10k on Saturday. Took a Camelbak and drank far more than I normally would. Got round in 1h 50m which is well off PB pace but fine for a training run. I was heavy legged and a good 30s per mile off of where I would expect to be.
Not very scientific but I reckon it's fine to run within a couple of days after giving blood provided you hydrate well and don't expect to be fully on pace.
Work got in the way of the Friday run so the scenario is .... gave blood Wednesday morning and finally got out this morning so skipped two days. I did a regular 10k route in just inside 50 minutes. Probably 3 or 4 outside what I'd normally do it in and nearly 8 off personal best. Some of this is down to having recently come back to running after a heavy cold but I definitely felt heavy legged and was very dehydrated - I don't take water with me on a 10k but normally need a few gulps when I finish. This time I drained a 500ml bottle.
So I reckon a day or two off is a good thing after giving blood and not to expect to be back at peak too quickly. I'll see how it goes but I am also still recovering from the cold so that's bound to be having an effect. I'm due to go out and do 12 miles tomorrow, which wouldn't normally be an issue but I was wondering if I had it in me after the 10k ... I'll report back ....
I'm giving blood Thursday morning and was planning to do 5k in the evening ... I can live with 10% off given where I am in the training schedule. I'll listen to that MarathonTalk podcast on the way round!