I'll second what Ben said. Initially, spend your time building up your ability to handle the equipment you'll need for the truly big animals. Once you can handle the equipment, learn to USE that equipment; learn to be comfortable using it to hunt with. I hunt doves, rabbits, fox, and everything else with my buffalo setup!
Spend your time and effort (and money) on getting the most from your arrow setup. The right arrow will let you get away with using a bit less bow force or, looking at it the other way around, it gets the maximum possible from whatever the heaviest bow that you can use effectively turns out to be.
I'd also recommend that you get into doing some predator calling - even if it's predominately just foxes. This is an exciting, and often tense form of hunting but, most importantly, the predator responding to a call is among the most difficult of animals to get an undetected shot off at. They are expecting some other animal to already be there; so they are alert and looking. Learn to beat them with any degree of regularity and getting controlled shots off at most other animals becomes a lot easier.
Third, take on some pigs; but do it from the ground, by stalking. Don't shoot them from blinds or stands. Try to get as close to them as you can before taking your shot. This will get you use to working close, and to an element of potential danger.
Forth, move on to some black bear hunting from the ground - and not from inside a blind. I've shot baited black bear at a range measured in feet; not yards; and it can be as nerve racking as shooting any buffalo.
If you have the chance, do a bit of moose and elk hunting. It will help you develop confidence in your setup's performance against sizable game. Along that same line, on every one of the larger game animals you do kill, take the time to do some test shots into those 'tough to penetrate' areas. Dissect the animal to learn exactly what your setup did; and what it didn't do. In essence, learn what your setup is capable of doing, and what it isn't. If your setup won't shoot through pig shoulder bones, it's not enough for the ribs of things like buffalo.
Spend a lot of time learning to stalk properly. The best of the big game hunting comes through stalking. For things like buffalo, stalking is a 'must have' skill.
Hope that helps a bit.
Ed
TGMM Family of the Bow