Well... I wouldn't donate blood a week before a race. They say you don't fully replace all those blood components for 7-8 weeks. But this article suggests you *might* recover from the loss in 1 to 2 weeks. And I assume a long-distance event like a half would be a submaximal effort. But I know I've felt pretty crappy just jogging easy a couple days after donating.

A study (3) on the effects of blood donation on 10 competitive cyclists before and after donating 1 U of blood found that maximal performance was decreased for at least 1 week after blood donation. (Cyclists were measured at baseline and at 2 hours, 2 days, and 7 days after phlebotomy.) Although researchers found that maximal performance was decreased, submaximal performance was unaffected.

http://www.physsportsmed.com/.../2001/06_01/news.htm

Stay well hydrated!