What will running for 45 minutes a day 3 days a week do to your body?

You will obviously be tired at first, but it will get easier as you go along. You will also lose weight, have a better heart and lungs, feel better about yourself, and it will realy tone your legs. I would not run as long if I was just starting out. Try just 20 minutes at first and then add another 5 minutes every two weeks.