Stand at arms length of the heavy bag, and throw straight lefts and rights as fast as possible for 15secs, then 15secs rest, and do that for like 3mins, its real tough/intensce and increases you stamina real quick, i think the bag heights ok, you punch shoulder height anyway, even against a taller fighter. Keep up the good work, and remember to keep your gaurd up even while on the bag to help develop good habits