[1] And if some kind of tasks are too brutal, there's always swimming (or even qigong).
Lately, I've been experimenting with a split schedule. I work 8-12 in my morning time zone solo, then I work 5-10 with a 1 hour dinner break in my evening. This gives me 3-4 hours of pairing (depending on how my partner and I schedule lunch/dinner). Between the hours of 12-5, I do some cycling, go shopping, do some cooking, and importantly have a 1 hour nap.
While it's not ideal (would love to get in a few more hours of pairing), from my own personal perspective it has been extremely good. I am able to concentrate fully for the entire day. I never feel tired at work any more. The nap is amazing!
Of course, it is 11 am and I'm writing a post on HN (bad Mike...)
Sometimes I do a fair amount of training / mentoring for junior engineers as well. That can be pretty relaxing too and productive for the company.
Working as a programmer now, I find that I mostly can put in about 4 hours of real work in a day, before my brain is mush. Although there are other blessed days when I get in the groove and have plenty of work to do with no interruptions, and I might work steady for 12 or 15 hours straight. It takes something of a miracle for this to occur, though, since two or more 30 minute meetings, or having to bounce around a couple different projects doing email support can wreck any attempt at getting into a flow for the day.
Physical labor might be strenuous, but I generally find it to be less exhausting than mental labor.