It is less a question of how hard it is to work up as it is how long it takes. To move up in weight you need to be sure to loosen up well. This can be done with exercise, shooting your regular weight bow, or perhaps a combination of these two before grabbing the heavy bow. Shoot a relatively few arrows at first as you improve condition. There is no need to get it in a big hurry. Shoot only the number of shots that you can reasonably control. When you get tired, stop as accidents can happen and form is lost. As you shoot over time, you will pleasantly find that you can control the bow and maintain form for a significant number of shots. As even more time progresses, you will be able to shoot more and more arrows and still maintain form and control for a full shooting session. The trick is to follow Dirty Harry's admonition, "A man has to know his own limitations". How long? Well, it takes how long it takes. For some its longer than for others, but don't be intimidated by it. Although, I was younger then, I went from 58# to 78#, and yeah, that took a while. However, it was not that hard to do, but one last time let me emphasize that a big jump in weight needs to be achieved over time, not just a couple of weekends. Sorry about the long post, but I want to be totally encouraging in your effort, while making sure you know to take it slow and easy.