When I am running reasonably high mileage (for me, that is in the 80-85 mpw range), I am generally running every day. But because of my job, I occasionally am forced to take 2-3 days off at a time. This only happens 3-4 times a year, but it sucks when it does.
Now, I would expect that when I resume training, I should feel pretty good from the enforced mini-layoff, but the opposite happens. My first couple runs (first one especially) are terrible. I feel like I raced a marathon the day before (or got hit by a truck, seriously), even though I haven't ran for three days. Totally winded, stiff, sore, and unable to get any sort of rhythm going. It usually takes 2-3 days to resume with feeling normal. Why is this? I'm in my 40s, so that might have something to do with it, but if that is so, then why do I feel fine putting in decent mileage day after day?