Difficult to put numbers on it 70-90 is a bit of a jump. Most reckon 10# steps is sensible. Those rubber band trainers are good. It's a matter of listening to your body, but it's an odd thing, I find it easier to draw a bow with an arrow on the string, some tell me they find the opposite.
There is a point where you just can't get to full draw, but once you do it, then you find it relatively easy, there is an element of confidence as well as technique. It helps to have some lowish poundage, but long draw bows, so you can get the feel of the technique.
The mechanics of drawing with your drawing hand high, works as follows:- If your right hand (assuming that's the one drawing back the string) comes back high, as if wiping your forehead, it (and more important, the elbow) then moves in a long downward arc of say 12" yet this only draw the bow back an extra 4"... this is effectively giving you a 3:1 leverage, like one of those over centre clips/clamps.
Hope that makes sense and helps.
One exercise I found good was to lean back into an open doorway with your elbows out at shoulder height. Your elbows press on the door frame and stop you falling backwards... shuffle your feet forwards a little so you are leaning back more (don't slip!) and then use your back muscles to force your elbows back and your chest forward. It gives that feeling of engaging the back muscles.
Any exercise that improves shoulder strength and mobility helps.
This post from my blog shows me almost getting the 90pounder back. It just need that bit more, a higher draw and getting the elbow over the top... a bit like getting a compound over the hump...that's effectively what the high elbow does, it acts as a cam.
https://bowyersdiary.blogspot.com/2012/01/shooting-90-longbow.htmlDel