How important is exercise for PhD students?
PhD in most science streams (I don't know about other streams) is hard work. Long experiments done on your feet, late night +Early mornings etc. You can not do this and remain lucid enough to make sense unless you have a healthy body. So exercise and healthy food is a must. Otherwise you will not be able to keep it up for 5+ years the degree takes.
In Earth Sciences you are also expected to do all your field work and help other students in theirs. Hence, most Earth Scientists are almost officially required to be of athletic level fitness no matter what they look like (rarely do they look like Serena Williams or The Rock).