Today I have had the worst run ever. I have done one 25 minute run, but today I was flagging after 5. I couldn't get a steady rhythm and my breathing was very tight. I stopped running after 7 minutes and had to walk for ages before I felt right. I did another 8 minutes after that and had enough. The only thing different is that I gave blood on Sunday. I've been doing this for 40 years and never had a problem afterwards but then I've never been pushing myself to the limit afterwards either. They advise 24 hours before vigorous exercise so I thought today would be OK.
Has anyone else had a similar problem?