Will I stay fit if for 2 days a week I lift weights and hit the punching bag?

Well sure, you'll probably maintain your level of fitness, but if you're looking to get toned and build muscle, try lifiting weights 3 times a week. Also, hitting the punching bag can be considered cardio, but you should really try running or biking several times a week as well (again this is assuming you want to increase your endurance).

Try and do at the very least 30 mins of cardio (which would be the heavy bag) Then you'll be in good shape. ADD: I alternate my cardio days and my weight days. That works for me. 15 minutes on the bag really isn't going to do much but get your blood pumpin' for a minute. Cardio sucks I know but it really can't be avoided and I can't stress how important it is. You can lift weights every other day for years but without the cardio your muscle is going to be hidden under the fat. Trust me!