Really interested in the questions FallDuskTrain and wolfgaze have posted. I struggle with similar issues around wanting to do more than I actually can. Right now I don't work full time and still can't fit it all in.

I find it easy to procrastinate if I am not busy enough but I can't bring myself to go back to being too busy after burning out some years ago. I am more idle than I would like but busy by other people's standards.

You mention wanting to build up endurance, but is that what you really want?

I find this post about time thought provoking but also terrifying: https://waitbutwhy.com/2015/12/the-tail-end.html