use footworking , evading and slipping all while throwing straights, jabs and combos and youll work your legs.....also learn some good drills to help with your wind
http://www.rossboxing.com/thegym/thegym24.htm ....running is probably the king of all cardio but can put a lot of wear and tear on the joints over the years ....if you just want to get fit and "knuckle" tough then the heavybag is probably the most valuable tool there is. as for stomach fat? watch your diet a little better .