My wife has pretty much NO sex drive, and she used to try to limit me to half an hour of karezza, once a week or less. A few months ago we changed how we do karezza, and since then she's been willing to do it for about an hour, once or twice a week.
What we do now is: I enter in missionary position (it's easiest for me that way). Then we immediately swing one of her legs over my head, and we go into bridge position. Then she does Kegels, squeezing once every 3 - 4 seconds. She does that continuously for about 45 minutes. I find it kind of amazing that she has so much stamina, and she doesn't mind doing it at all.
Meanwhile, I just lie still and bliss out. I call it "motionless karezza." I make some sounds of contentment, to let her know that I'm enjoying it.
When she finally gets tired or bored, a couple times we just ended it right there, and I've been satisfied. But usually I will start moving to ramp up my arousal level (from about 3 to 5 from the motionless karezza, to around 7 or 8 with me moving). Usually I'll then ask her to turn on her side facing away from me so I can spoon her. I may move some more, then lie still and enjoy holding her while I wait until I soften up and fall out.
I find it really hard to get answers from my wife about what she likes about sex (not much, it would seem ). So I don't really know, but I'm guessing the reason she can tolerate the motionless karezza for so long is that the Kegels give her something to do, something to occupy her mind, instead of just lying there passively, being bored with something that she has no interest in.
I'm pretty happy with this new way of doing karezza (compared with the old way where I did all the "work"). My arousal level is lower with the motionless karezza, but it's still enjoyable, and I certainly like the longer duration and more frequent karezza sessions.
About the medical issues: My wife has a sports injury which makes one of her hip joints uncomfortable if we have sex in the missionary position, with her spreading her leg out to the side.
Meanwhile, although I still have pretty strong erections sometimes when I wake up, they go away within seconds when I get up, and then it's somewhat difficult to get an erection again, and it's easy to lose it again. I've seen my doctor about it. He says it's pretty normal for someone my age (60's). He said he could prescribe some ED drugs. I declined. I can get an adequate erection by relaxing and cuddling with my wife for a few minutes, and/or by "fluffing" myself.
So, missionary position works best for me for soft or less-than-hard entry. Then we immediately go into bridge position, which doesn't cause discomfort for her hip joint, and also works very well for me, because the way we are pressed together in bridge position, I don't fall out even if I lose my erection. And with the continuous Kegels, I stay hard to semi-hard.