For both mine and your benefit, I want to outline the reasons why exercise is critical to being a successful scholar.

• Scholars live in our heads.  That particular muscle is working constantly.  But often it needs a break.  Doing something physical can provide that mental break wherein we sit back, let things process, listen to the music and do something else for a while.
• Scholars need time to process our thoughts.  As long as we are taking in new information or trying to force ourselves to write we can’t necessarily process what we’ve learned.  I find that long bike rides are GREAT for processing things.  No structure, no goals.  Just let the thoughts flow.
• Scholars need to periodically clear our head.  On the other hand sometimes we need to just stare at the scenery and not think about our work.  We need to be away from it for a while.  Again, for me a bike ride does the trick; I think about nothing but the scenery, my pumping legs and the music on my iPod Shuffle. 
• Scholars need a new perspective.  Looking at the world from the elliptical machine or the weight bench is different from looking at it from our desk or our favorite reading chair.  We see different things, different people, and sometimes get sparks of ideas.  It may or may not result in some deep insight, but the change is a positive thing. 
• Scholars need to give our eyes a rest.  It isn’t good to spend all your time looking at one distance, be that a monitor or a book.  Your eyes need the break as well.  Yes, you still use them while working out but you are suddenly using them differently.
• Scholars are stressed.  “Publish Publish Publish.  Oh, and maybe teach a little.  A little service would be good too.  But mostly PUBLISH!!!”   I’m still in grad school and I’m already feeling the pressure.  (I’ve been told that in my area you can’t get a decent job without several publications to your name….) For me, cardio (cycling) tends to reduce my stress, but I’ve also found Yoga very helpful.  (I really like Bikram, but just don’t have the time.  Instead I download the Yoga Today Podcast and do one of those when I have the time and need to relax.)
• Scholars sit on our butts for a substantial portion of our careers. 
  Because our work is reading and thinking and writing we spend a great
  deal of time sitting still.  Teaching is one break from this but it
  isn’t enough.  It is painfully easy to gain weight *blush* when one
  spends a lot of time sitting and thinking.  Exercise can help.

Losing weight would be nice, but it is not the only reason to exercise and I’m fairly sure it shouldn’t be the primary one.  These other reasons are just as, if not more, important.