Swamp Bow, Technique is important. A relatively small man with proper technique and practice can out draw a muscle man, with no technique and practice, any day of the week. If you are determined and, shoot a lot, these things will come to you as naturally as learning to walk. I have never found one exercise that can truly prepare you for shooting a heavy bow other than shooting a heavy bow. However you have to get yourself into some kind of shape to even start on that journey. One thing I found when I was starting out is that there is no one set of muscles that you can develop alone. It has to be the whole body, even the legs. A form of exercise that, to me anyway, was as beneficial as anything is the simple push-up. You carry the equipment with you and can drop and crank out fifty or more in less than a minute, done throughout the day, and varying the placement of your hands, far apart, close together, ahead of your body mass, on flat hands, on your fists, the variety is endless. Always use good form and technique, slacking only cheats yourself. The thing is you don't want to build one line of muscle, spread the wealth. You can do it anywhere, who cares what people think. Another thing I use to do was grab two or three bows of lesser weight and practice drawing, its terribly uncomfortable, but makes handling A single heavy bow seem so much more manageable. I hope this helps a little, and good luck!
Robby