Very true. When I ran a marathon for the first (and possibly last) time in 2013 at the age of 43, I had to force myself to go more slowly than I was capable of running in the opening miles, because I knew I wouldn't last the whole 26.2 otherwise. That strategy - plus years of walking and running on San Francisco hills - paid off, and I was able to finish in just under the six-hour time limit. Unimpressive by competitive athlete standards, but a significant accomplishment for me.