> I tend to stop running just because I want to work on something more mentally stimulating and tangibly productive

Maybe accept that running, or perhaps exercise in general, is not your thing? You clearly have more pressing interests and there are other ways to maintain good health. My grandfather lived well into his eighties with nigh a scrap of fat on him and, near as I can tell, did no formal exercise after his army days ended. My mother is on the same track despite health issue severely limiting her mobility.

I say this as a guy who rides an indoor cycling trainer 3-4 times a week during the winter and turns in to a raging jerk if he doesn't get his workouts done. If you're not a "gym guy", "runner guy", "biking guy" or whatever, why waste your extremely limited free time trying to become one?