I think the only way to shoot a heavier bow is to shoot a heavier bow, shooting 100 arrows with a 45lb bow won't make you capible of shooting 70 pounds.
One good way I've found to work up in weight is to buy some of the less expensive older bows online - maybe 5 lb increments over what you shoot, and use them to work up. Of course keep your form good, draw carefully so you don't hurt muscles, reach and hold a solid anchor, and when your ability and accuracy level improve at each weight, move up.
As mentioned earlier, barring health conditions, most adult males can shoot 55 pounds proficiently, and probably work up to 65 - 75 without working out every day.
A lighter bow will absolutely kill most game on most shots, but many people shoot more weight better (easier to release and better back tension) once they build into it, and a the more weight permits you to shoot a heavier arrow, thus more penetration if you do happen to strike a shoulder or something.