I had a very strange experience MTB-ing on Saturday, a few hours into my ride I lost it physically. Suddenly, I found it very difficult going and I couldn't keep up. My legs wouldn't turn fast enough. On top of that I had cramping in my calves and I could barely flex my hands and fingers. While I'm not half as fit as I want to be, I know I should have had more in me. There were people I'm normally in better shape than who I just couldn't keep up with. It turned into a very disappointing day.
Somebody suggested my problem was I'd only eaten a bowl of cereal before I came out and I simply ran out of fuel in my body and lost it. Is that possible?
Related to this I really want to get stronger, and was wondering if going for a running one or two nights a week when Im not cycling would be a good idea to increase all over fitness/cardio?