A couple of weekends ago, I decided to give the local cycling club another try, thinking it would be a good idea to build up some cycling stamina and do some longer rides above the 10-20 mile utility journeys. I turned up and decided to go with one of the slower groups, advertised average speed 12-14 mph, as I didn't want to overdo it given I haven't cycled 40 miles in one go for many years. It was a lovely warm sunny day, keeping up with the group was no problem (I typically average 13-16 mph on rural utility journeys), however the group did the 40 mile ride at an average speed just below 16 mph. I felt fine after the ride but it was after I got home and had lunch that the problem started. I quickly felt a wave of fatigue wash over me and felt like I had little energy to do anything, so I went to bed for the afternoon, emerging late afternoon as I wanted to go to the garden center. The plan to do a bit of allotment gardening had gone out of the window, as I was too tired. What causes a delayed onset of severe fatigue after a couple of hours of moderate exercise which wasn't taxing at the time, and is there anything I can do to reduce the chance of it happening? I am reluctant to go out on regular club rides if it ends up wiping out half the weekend. I had plenty of water to drink, on the ride and when I got home, so it is unlikely I was dehydrated. This didn't happen a couple of months ago when I went on 7 hour walks in the Scottish highlands.