One nice thing about fieldwork is that all the tedious dragging around of heavy equipment tends to keep you in shape. Also, I'm somewhat of a spaz and I spend most of my time in the field in motion. A couple of observations:
I used to swim competitively in high school, but my swimming and other formal exercise activity dropped off afterward. A few years after I started working, I started swimming again. To my joy, I found I was able to kick ass at the butterfly, which I could barely do 2 laps of in high school. Yay shoulder/back muscles!
I'm fairly skinny, but some of my relatives are very delicate-looking. Someone took a picture of several of the ladies wearing bathing suits at a family function. I loomed over my relatives like some sort of giant linebacker, thanks to my (relatively) massive shoulders. I regret to inform you that this picture has been "lost", so you'll have to take my word for it.