Originally Posted by
Timtruro
I don't think you lost that much fitness in one week. You may have come back too soon, some of the weakness from your illness probably lingered. It is not unusual to feel weak or out of it on the first day or two back. Take a day off between rides, pack in some protein and stay hydrated and you will be fine for the long ride you plan.
+1 You were probably looking at the weakening effects of a virus.