61 here. Started riding serious last year. Was a slow runner and a bit of a gym slob prior. Am a highly consistent D grouper (lol). I have a low end bike computer with a HR monitor...for me, I watch and try to maintain HR and cadence, the time from point A to B is not a factor. This year I'm working on my "stand to climb" as that looks to me to be where I can make the most gain, if any.. If I'm burned out for the day, I go home or don't ride. There have been times when hitting the wall that it was just too high. Note, in my case, I can burn myself out on any ride by pushing to hard the first ten miles...it takes that long for my metabolism to fall into long haul let's work mode. Nutrition and hydration are key to the feel good as well. AC/DC...relax? C'mon man, where's the Barry Manilow? Lol. Keep smiling.