Will it help? Yes most certainly. Will it be fast? Not as much as you hope. First off a heavy bag is very hard to keep wailing on for hours and hours. Think I am wrong try it out at a gym that has one going for 30 minutes all out is incredibly difficult. Also you are not working out your largest muscles aka the ones in your legs.
Try jump rope (thats what boxers do on top of heavy bag training). Jogging is good. Burpees work if you have the ceiling clearance. I personally underestimated my jumping ability and jammed my fingers into the drywall.
I hope this helps