I'd like to be in this for the long haul and have the ability to do intensive retreats.

The main physical problem that seems like it could prevent this is a chronic back problem. I searched through the forums and scarcely saw back pain mentioned, so I assume that most people meditate without 'long term' back pain from sitting, presumably without going out of their way to avoid it. I have seen claims that long meditation sessions have contributed to chronic back pain too though. I've had some lower back trouble in the past, so I assume that makes me a bit more prone to having a chronic problem.

What's the general risk of a chronic problem or what generally factors into the risk? Plenty have done 90+ day intensive retreats. This seems to indicate that meditation is very low risk if most of them are doing okay. Did their bodies have to 'work up' to that kind of time? Or can pretty much anyone just start doing sitting meditation 5+ hours per day without having their back develop a problem?

What would be the best ways to prevent this? For example, certain yoga postures, strengthening exercises, a good book on improving posture.

strong core helps reduce back pain for one. if you do crunches each day I think it will likely help.

Sensitivity and responsiveness to spinal discomfort.

what does that mean

I already have chronic back pain through a repaired L2 disk. That and the surrounding disks are now desiccated so my back is pretty much busted. However, my back surgeon told me that I could use tolerance of pain as a primary factor as to what I should or should not do. In other words, I'm not going to do myself any more damage than I've already sustained. So if I can tolerate it, I can do it.

So I'd take from that, that the primary concern here isn't that you're going to wreck your back, but simply that you may get sore or tired more or less quickly depending on how you sit. Me, I sit on a chair. I've tried a seiza bench, which isn't too bad. But a cushion is impossible since my knees just won't go anywhere near the floor. Someone at the local Zen center suggested I prop up my knees with some small cushions, but if I did that I'd have a pile of cushions either side of me about 18" high! I'm not sure the Zen center actually has that many cushions :-)

"Treat your own back" by Robin MacKenzie is the go to book for this stuff.

I'm no expert, but I used to have back problems. I'd say they were due to both 1.tension held in the back and 2.weak muscles. Usually my back was ok, but from time to time I'd lift something, or twist funny, and put my back out for up to a week or more. I'd be in a lot of pain and discomfort. About 11 years ago I saw a chiropractitor who recommended I try Pilates. I didn't go to a class but found a Pilates video instead. I followed the very simple instructions to the introductory exercises (about 20 minutes) each day for several months, and my back improved tremendously. Pilates is very core-strength building and relaxing. Since then I've had few problems. Even if I accidentally twist my back and end up in pain (this happens about once a year), it's not too painful and it heals in a couple of days. It's only my knees, ankles and buttocks that complain if I meditate for too long! (But I don't sit for longer than 1.5-2hrs anyway). I don't do Pilates anymore because I'm now more into qigong (also excellent for core-strength and relaxation).

So find a regular exercise to both 1. relax and 2. strengthen the back. (I don't think just core strength on it's own is enough, though I'm no expert!). Personally I'd highly recommend qigong, but with a good teacher, as they can point out poor posture and where you're tense (see above link).

It might also be a good idea to have an expert check over your back.

Shift when it gets uncomfortable.

I think most doctors say the same thing. I sit in a chair but I used to get back pain if I didn't exercise. I would recommend laying flat on your front and using your back to lift your head and torso up and down. Pushups and crunches also help because muscles support your bones.

Thanks everyone for the responses. It sounds like in general, pain in meditation won't become long term and is can be sat through without much concern.

I started doing a qigong routine about a month ago, so that ought to keep me in shape to do plenty of meditation. I may check out either pilates or yoga too, to isolate working on the back until it's a bit stronger.

I had this problem to some degree; I asked a yoga teacher and he told me to make torsions... they helped a lot, now I don't feel it any longer; something like this before a meditation session was more than enought to overcome the issue.

Yes I do a similar stretch once or twice (left side and right side) for a few longs breaths after each sit too. It certainly feels good.

Not sure if you've already found a cure for your back pain, but maybe my answer might help someone here.

A few years ago, I was short on cash and ended up buying a fake mattress. My previous mattress was no longer supportive and I therefore needed a replacement before I started developing back pain. But my biggest problem was oversleeping because the mattress was not comfortable at all. The new mattress ended up being worse than my old one. After days after sleeping on it, a dip had already formed. I regret purchasing it because I ended up developing back pain.

The good news is that my back pain didn't reach to a point that I would lose my mobility- but it did prevent me from enjoying myself or even having a good night's sleep. I've since bought a more supportive, high quality mattress but I have surely learnt the hard way.

But one thing I've learnt is that you can get back pain very easily if you don't learn how to properly protect your back. I noticed you learnt about long meditation sessions causing back pain. I've meditated a few times and I can assure you that there is a risk of developing back pain. That's why I bought a meditation chair with good back support. Of course, exercising regularly and performing certain yoga postures can help, but its important to use a meditaiton chair, rather than just using a mat. 

One thing I know is that you should never compromise on your back health. I am witness that back pain can be disastrous and devastating. Another thing I regularly use to protect my back when perfoming chores is a back support belt. I use it when lifting heavy things, when lifting weights, or basically when I need some good support for my back.