Last weekend during the day I went out for a ride, planning to do about\n20-25 miles. It didn't take long before I decided that wasn't going to\nhappen, I felt knackered, every press of the pedal just felt heavy. At\nthe end of the ride I told myself I was probably getting too old for the\nlong rides I used to do and probably managed about 5 miles.\n\nToday I went out planning to do about 5 miles but the more I rode, the\nmore energy I seemed to have. So I just kept going. Eventually I seemed\nto be able to cycle fast uphill and not even feel tired doing it. I\neventually had a feeling like I was running out of energy (but not\nfeeling tired, if that makes sense) but felt I could just keep going if I\nwanted to. At that point I did return home and I did just over 26 miles.\n\nI don't get it. Sometimes I can really push myself hard and my body\ndoesn't even complain about it, other times I can barely get off the\nstreet before I feel knackered. I'm not sure if it makes any difference\nbut the time I seem to do well is late at night, say between 8 and 10pm.\n\nAny ideas?