absolutely don't give up Deb. I'm not a good cyclist (just starting to get more serious now) but have been an avid runner for over 12 years. Throughout my training, I can tell you that there are just some days where I feel "off." Also, I find that if I run, say, 4 days straight, and then take one day off, the run after my day off is often with "deadish" legs. I used to get upset about it, but now I just attribute it to my muscles rebuilding. Sometimes it can also be a lactic acid issue. At any rate, just take it easy on the day you feel off. Plug away and I am sure you'll be even stronger the week after!
Trying to be the person my dogs think I am.