Just look up what you can burn with sports; that should be about how much energy the body can provide. For example, looking at this link I get that by running you can burn up to 22 calories per minute (however that table may not be complete). Therefore the body should be able to provide those 22 calories per minute. However since the calories burnt depend on body weight, the calories that can be provided may depend on it, too. It might also depend on your training status; information sites targeted at professional athletes might also be a good source of information.
Note that at the end of the linked site there's a calculator where you can calculate the energy you burn for different types of sports depending on your weight. This should give you a good feeling about the available energy.
You'll also have to define how efficient the energy transfer from the body to magic is (that is, how much of the energy you draw from your body actually ends up in the magic). For example, the muscles are not too efficient for converting energy into work (according to Wikipedia between 18% and 26%); a lot of the energy you put into them ends up as heat, which is why you sweat when you do sports. So if you assume a similar efficiency for magic, the 22 calories figure above will only amount to between 5 and 6 calories available for magic.
If using body energy for magic also causes waste heat, then it might well be that the ability to get that waste heat out of your body is the limiting factor. That might even mean that a magician can do more magic in cold climate.