Low performance

In the past week I was give blood. Everything goes fine. I just can't do any sport activity during the 24 hours after it. In the weekend, to days after, I was feeling fine and so I did my first run. It was to be an easy 40 minutes run. 10 minutes after my start, when I begin the first road ascent, I was tired. The remaining run I was so tired that almost have to stop...completely tired. but no, I go to the end very slowly but without need to proceed walking. I never thought that is so difficult to run with a little less blood in the body, but this is a amazing difference! This week will be a couple of easy runs to progressively becomes to the same point I'm before!