I know the official word is it takes 8 weeks for the system to totally recover, meaning you can donate again in 8 weeks. I donate once a year at the end of January. I knew my workouts would be affected, but it's still frustrating when I get tired out after a couple of miles. Is it better to scale back speed, distance, or both? My standard run is 3 miles - I did a 4.5-mile "run" last week but it included walking breaks.
Guadalupe River Run 10K April 2013 (1:01:36) PR!!
Turkey Trot 5K Nov 2012
RSS Fun Run 10K May 2012 (1:03:17)
Mermaid Run 10K May 2012 (1:09:37)
Turkey Trot 10K Nov 2011 (1:05:20)
Firefly 5K Oct 2011 (49:14) - with DH and DS at son's pace
Blood donation will have an effect on your running for a while after you give (days to weeks). It does vary person-to-person. You will probably feel sluggish for a few days to a week after donating. It will take about 24 hours to replace the fluid but the hemoglobin/red cells take a few weeks (typically 4 weeks for red cells, 8 weeks for the iron). And those are the O2 carriers. Proper diet helps but it still takes time. I have run a pretty good race (up to 10 miles) about 2 weeks after donating. I'm usually good for all but the very longest runs within a couple weeks.
Here are a couple of posts that I wrote that might help you. If you feel tired after donating, I would reduce your speed, rather than your distance. Your speed is effected more by blood donation than your endurance.