@stech
Unless you are naturally a big, strong guy, you are not going to reach 80lbs. plus of draw without some sort of strength building routine in my opinion. In fact, the specific muscles and tendons used to draw a bow are rarely developed through routine exercise and need to be developed through specific movements. I have guys I work with who are amateur body builders who could not draw my 72lb. every day hunting bow back on the first try simply because they had not developed the muscle group specific to that movement to the extent that I have over years of shooting.
The quickest way to get there in my opinion would be to use the proper combination of strength building and flexibility using a few simple exercises. I would recommend using something similar to the Smith machine (below) at your gym. Any combination of push/pulling exercises that you can do on the Smith machine for your back muscles and specifically your latissimus dorsi will get you there the fastest. Seated rows starting with close-grip, and working out to wide-grip have worked best for me.
In addition to the back exercises, I suggest good old fashioned bench presses, or push-ups if you don't have a bench. The draw cycle, done in correct form, is a pushing/pulling movement. The strength for the movement comes from your chest and tricep on your grip side, and your lateral back and shoulder on your drawing side.
If you don't have machine similar to the Smith, you can also use dumbbells and the exercises below:
I also recommend using an athletic rubber band to stretch and warm up for several minutes before doing the serious weight. I do a couple dozen push-ups , then several different stretches from various angles with the rubber band first. Just an FYI as a final thought.... Drawing 80 lbs can be achieved relatively comfortably, but drawing 90-100lbs is going to be tough regardless of how strong you get. Your goal should be the ability to draw and let down several times in a row if need be without excessively contorting your body to get the bow back. Good Luck! Hope this helps get you there!