It's hard to say, really. I was thinking about it a little today (on my run in the rain). I wondered if maybe it was providing some balance... some physical stress to accompany the psychological stress induced by lack of research, or real sense of purpose. Sure, I want to get a Ph.D., but to what end? To go teach? I've wondered aloud before about the point of teaching. This is a fairly cynical viewpoint, coming after several tedious hours of tutoring and weeks of no progress in research. But I realized some other point about running. It's the sense of accomplishment.
Consider, if you will, my day today, which for the sake of argument I'll claim to be vaguely typical. Let's say I spent about 2 hours reading rss feeds and things, 2 hours in class, 4 hours tutoring (only scheduled for 3, damn guys), more than an hour walking to/from class/tutoring, 1.5-2 hours on "research", and about half an hour running. These are my "accomplishments" for the day. Of them, the running is somehow the only one that I actually feel accomplished anything. In any given day I can stare at my research for several hours, and not come up with anything, which is what seems to happen most times I actually sit down to do so. The tutoring... I get paid for, but I don't know how much of an impact it has on the mathematical development of the students most of the time. And it takes away from my valuable research time/energy. Classes... well, they have their moments, and my rss feeds I enjoy reading, but I generally don't feel like I've accomplished much (even if I 'catch up' on my starred items).
But with running, every day basically is an accomplishment, or seems to have an obvious correlation to an upcoming goal. Like today, I ran 4 miles in 25 minutes, plenty less time than any of the activities above. But today was one of my best paces, and comes after a 16 mile (1:50), and 10 mile (1:06) day. So distance and time-wise, I realize the accomplishment. Even on less of a running accomplishment day, I can feel like I did something, and I know that'll it'll help me with some other running goal or upcoming event. While I'm feeling good about having accomplished something, here at the end of the month I thought I'd see how much running I did in the past month. I covered about 170 miles in something like 24 or 25 hours. In all, that's probably the most productive 24 hours I had in the past month.
What's my point? Looking at my other posts, you won't be surprised to find that I don't have one. I seem to think that when I started this post I had something more to say. I guess tiredness crept up on me. And then I posted this and realized I should have included some links to some comics: 1, 2.
I guess I can suggest for beginning runners (which I still consider myself, I've only been at it 2 years or so) to keep a log of your runs, with distances and times. I use mapmyrun, only because I've not quite written my own version of it yet (more on that sometime). And have an event you are training for. It doesn't matter how far or how fast you are running, there is always somebody else out there who will beat you (Ok, strictly I know this isn't true. But in practical application...). Just set personal goals, and go out there and have fun. And heck, if you can beat the guy next to you, or just pass somebody on one of your training runs, that's nice too.