I just ran a trial 5k on a treadmill and my time was 27:42. That's my fastest time in over a year. I've been sick with diverticulitis and the antibiotics that I'm taking to fight it, so that's saying a lot about my overall fitness level. My actual race is on the Fourth of July and I will have been off the drugs for five days--my last ones are tomorrow.
As I neared the end of my run today I remembered something about one of the drug's side effects. Cipro is one of the drugs and I remembered that one of the side effects is to more easily pull a muscle. I could feel one going in my right calf, but wanted to complete the trial run, so I pushed on. It's a bit sore, but it will mend, and since I'm going to be tapering off in preparation for the race, I don't anticipate any problems by race day.
I guess the bottom line is that if you're trying to stay fit, push on through temporary pain with lighter workouts and you can pretty much maintain a base line level of fitness. Then, when the opportunities arise--like today for me--see what you can do. You may surprise yourself.