Sometimes when I do feel tired or have a headachy or my legs are kinda achey a bikeride helps me. Granted I dont do 40-80 miles but a 12-25 mile ride will make me or break me...
I do turn around when I reach my breaking point.
I try though because usually that will either work out the bugs or make me so tired that I sleep well that night and feel much better in the morning.
This winter I had the flu all around me at work and I stayed committed at the Gym after my surgery was healed and I didnt get the flu worse than a few sniffles. I really think that exercise and sweating and really working the brain and body keeps you healthy. A good diet helps too.
Now that gas has gone up I plan on biking to work everyday so I should be very healthy for the riding season....