I've regularly given blood and rode the next day to work. Now that's not a long ride but I haven't noticed any impact, nor have I noticed anything on the weekend (blood donations are generally on a Thursday for me). It might not mean anything because if I'm a bit slower than the typical pace I would be unlikely to notice it and chalk it up to another factor (wind, going too fast early, different route etc). I know they say no heavy lifting for 24hrs after but nothing about riding