The Benefits of working out -- in music?

I am a music student and have heard several classmates swear that running, or doing cardio right before a lesson/practice/rehearsal helps them play or sing better. I tried this right before my singing lesson today and I have to agree, I had a more open sound and I seemed to have stronger breath support.

Do you think that the musical ability of a singer or wind instrument increases directly after exercise?

i definitely think so.  i don't know anything about music in particular, but exercise opens your rib cage, expands your diaphragm, and forces you to maximize oxygen uptake.  it's like doing voice exercises.  this is an article most responsive to your quesiton:  http://www.musesmuse.com/00000293.html

here's an article that says musicians usually like yoga because it connects mind and body.  this is a brochure about music used in facilities that house people w/alzheimer's and whatnot, saying that singing boosted their cardio health bec, among other things, it releases endorphins (so sort of the reverse of what you're asking, but still interesting, no?).  so huff, puff, sweat and belt your heart out!

When I was studying music in college my trumpet professor told me to run a few miles a day.  for the short time that I actually stuck with it the improved lung function definitely helped with my instrument.

I believe there were some benefits as far as mental alertness as well.

As a musician, physical fitness is absolutely essential!  Music making is a very physical activity.  Our bodies are our instruments!

Most of us put on weight because we are compelled to sit for long periods of time for rehersals, practicing, lessons, and concerts.  So that's even more reason to exercise!  

Cardiovascular exercises and light weight training (too much/too heavy weights can be harmful to your hands - you don't want to pull a muscle or strain an arm!) as well as careful stretches before playing can vastly improve your performance.  For singers and wind/brass players, jogging and swimming gets your lungs in shape - they even grow! 

Bottom line: Exercise builds strong bodies and strong minds - we need both to focus on the musical tasks at hand.