Guys...jogging for 20 minutes is not going to burn muscle. If you are working out and start burning muscle-you are already in a starvation mode. Lance Armstrong in his Tour De France did not start burning protein (or muscle) as a fuel source until the last couple days and this is a guy that's on his bike continuously for 12+ hours for days and days and days on end.
There was an old rule of thumb-20 minutes is carbs, 20-40 minutes are carbs and fats, 40+ minutes is fat but it's dependant on each individual. Swimming is a good alternative to other forms of cardio because it's easy on the joints and requires more work or energy expenditure, which equals more calories burned.