I'm not convinced I have enough energy to do 16 hours a day of stuff that I am proud of.

Recently I have been doing several projects with my car in the shop.

I wouldn't say I was proud of the many many hours spent sitting in the shop and by any metric or observer, doing exactly nothing. On the other hand these hours were essential to success and the work would have gone better and been done quicker if I had spent more hours sitting doing nothing in close proximity to the work. (some time was spent very low productivity cleaning or arranging project related things, most time was spent quite literally sitting and apprantly doing nothing)