However, would I regret not being fit enough to go for a hike in my 60s? That's another story, and it is one which many do have the ability to intervene on.

Treadmill time as an enabler of other experiences may well be worth keeping, even if the time spent on it doesn't satisfy some imagined "will I think of it on my deathbed as time well spent?" bar. I imagine there are a great many things I won't think about on my deathbed which on their own are unremarkable or even gasp not very fun, but that is of course not a reliable measure of whether something is worth doing.