wilbur
I don't have a great handle on long rip cuts with a Japanese saw. It's like the final frontier of sawing for me, and quite honestly I think that's why God invented bandsaws.

But there are some things that I've tried that seem to help, although I don't know if they are at all traditional.

First, sawing on low horses is the way to go, unless your bench is ridiculously low. I know you have problems with your back, so try different heights. Maybe one will be more suited to you than another. It will be a very quick and easy job to prototype a low horse just to find a good height for you, and then you can build a nicer one.

Second, use two hands on the saw and use your legs. I find that sawing on low horses using arms only gets old fast, probably because I am out of shape. Regardless of why, I find that if you bend your knees slightly as you put the saw into position for a cut, and stand up slightly as you make the cut, you get a lot more power without giving up control. My guess is that if I put my legs into sawing, it's about 75% legs and 25% arms.

Again, I need to stress that I have no idea if putting your legs into it is traditional practice or not. But once I started doing that it seemed so effortless and natural that I think there may be something to it. Similarly, pulling a Japanese plane is as much about the legs and torso as it is the arms.