I jumped on that bandwagon faster than you could say "Go Cadel, Go!" and I was taking magnesium twice a day for the next two weeks and for a week after. In that time, I did a little research into this "supplement" and couldnt believe it had taken me this long to start taking it.
Magnesium is one of the essential salts that you lose when you sweat (tip: you do a lot of sweating when you run, at least I do) and it needs to be replaced. It also helps the body to absorb calcium. According to Livestrong, magnesium concentrations in the body change according to your exercise.