I commute to work Mon-Fri, 19 mile round trip with hills. I tend to find it progressively harder as the week goes on such that I am grateful when the weekend arrives and I can have at least one day of low physical activity. This weekend was different, however. I had the usual lethargic feeling on Friday but had to get up about half an hour earlier on Saturday morning because I was meeting someone 38 miles away and going on a short walk, which involved a train journey and a 19 mile round trip with hills, coincidentally about the same amount of climbing as on my commute. I expected to feel awful when cycling but for some strange reason felt strong both on the outbound and return trips. About an hour after I get home though, I get a rapid feeling of lethargy which persists through the evening, despite my best efforts to replace lost calories. Today, I had another walk planned, which was about 8.5 miles long in the South Downs, and involved cycling 18 miles there and 20 miles back. Despite having aching legs at the end of the walk and wondering for a moment if I should get the train back I decided to ride anyway and oddly enough, again felt strong (was averaging over 17 mph over the first 10 miles, which is quick for me). Again, I made sure I took in significant calories when I got home (in the form of oats, whey protein, peanut butter and milk blended together) but an hour or so later had an even bigger onset of fatigue which put me in bed for a couple of hours. I'm still feeling weary now. I've had this happen before and it was a reason why I stopped riding with the local cycling club, but over the last couple of years my tiredness issues have eased somewhat so I was hoping I would have gotten over this now. The phenomena is independent of what food I eat and sports recovery drinks made no difference. I would really like to understand what is going on here, as I can't believe other people who are physically active have to deal with this to the extent I have too (in fact I know they don't, going by some of the threads I have read on here). Does anyone have any ideas? P.S. I have considered going back to the doctor but I know the response will be along the lines of "What do you expect" and "It's just the way you are"