I have to put my 2 cents in....
I went from 85# at 28" to 102# at 28" in about 2 1/2 weeks. When I first got the 102#, I stopped short of anchor by about 3 inches. Wow, it kicked my butt.
However, I pulled that bow every day for about 4 days. I started with 1/2 draws and moved to 3/4 draws. After 2 days, I tried real hard to get full draw, but could not.
Then, took 2 days off.
Then pulled it again every day for 5 days. Then, took 3 days off - vacation.
Then I shot my 85# for three days straight. I shot about 350 arrows +/-.
I went back to the 102# and I was able to reach full anchor.
How did I train? Well, first off, I made it fun. Not a chore. I would pull the bow for 2 or 3 sets of 5 to 8 reps and then set it down. Then, every few minutes I would try and draw it again (1/2 draw and 3/4 - nothing that would cause an injury by being "Cold"). I did this for an hour or so while watching TV, every night.
After I got a little more "Muscle Memory" or "neuro-muscular overload" for that kind of weight, I could draw the bow to 1/2 or 3/4 anchor without much warm up.
Later on, I would just grab the bow off the rack, pull it 1/2, 3/4, and then try to go to full draw.
I turned 47 on Monday. So, I ain't getting any younger. But, I believe the consistent effort and desire to be able to draw the extra 17# was the key.
Last comment: I went to a local 3 D shoot with my 85# bow. One of the shooters that was with me made this comment, “not only can you draw that weight, but you can hold it, too”. What I realized is the 17# heavier bow made my 85# easy, in comparison.