I had to get a one day racing license to ride in the High Cascade 100 MTB race. I had no illusions of actually racing but I did want to ride the 100 mile course which was over 80% single track around Mt. Bachelor in Oregon seeing it as a great challenge. I never was more than a casual mountain biker (who used it to ride with my son when he was boy) when I did this at age 55.
I prepared for the race that spring and summer riding with 2 experienced buddies who had also joined up and was happy to be able to finish the course in 14 hours on what was easily the oldest bike on the course that day (Yo Eddy 26" hardtail). Man that was hard but also one of the most beautiful days I've ever had on a bike. I was so tired by the late afternoon that I began to hallucinate. All that exhaustion went away when I finally hit pavement and time trialed the last few miles to the finish line on pavement. I realized then just how mentally taxing all that single track riding had been. I haven't been on a MTB much since then but boy, what a great memory.