I'm definitely 'underemployed', but the nice thing is that the work I do is so far beyond what anyone else I work with understands that I report to the director of the company himself. As long as I get my work done, nobody really cares what I do.
I work until I feel like I'm not making progress anymore, and then I go for a walk. Generally the walk is about an hour long, but I've found that a 90 minute walk is pretty much perfect for resetting my mind and getting over any hurdles I've been stuck on.
The only reason I don't take a 90 minute walk every day is because my desk is near the staff who just punch numbers in to a computer all day, and they start to gossip if I'm not at my desk enough. I objectively finish more work and produce better work when I get up and move around when I get stuck on a problem.
The best way I've come to describe the effect of walking is to say it's a meditative act. I don't mean that in a spiritual way, I just mean it helps clear out the useless thoughts and gives you a way to center yourself.
One of the reasons I'm still working where I am is because I work in such a great area for walking. I worked in the suburbs for a while, and I'll never do it again. The car-centric design of suburban office complexes honestly feels like torture to me.