Did it! I was worried as I gave blood yesterday (good thing they no longer ask if you're going to do anything strenuous in the next 24 hours, just tell you to take it easy until tomorrow). I had a read on here a few days ago and there's a question about giving blood. So I followed the advice which was drink lots beforehand and after and I had no ill effects during today's run.
So, roll on Friday when I step it up to 2 x 8 minutes. (Still worried about Sunday's run though.)