About seven years ago, I found a one-man company who made a strap called the Y-strap. Unfortunately he has since closed shop. The brilliance of that strap was mostly in the simple idea that you put in on the shoulder opposite the camera, so it wouldn't slide off, and secondly that the camera was mounted in such a way that it would slide on the strap, so in less than a second you could grab it down by your hip and swing it up and fire.
This excellent idea has fortunately been continued by a slightly bigger company called BlackRapid. (Though I can't say if they knew of the Y-strap.) They make several models (admittedly some of them seem to push the raƮson d'etre thing a little), and they are all based on the same idea as the Y-strap. But they have added several features, most importantly a big, comfy stretch of padding which rests on your shoulder, so even with a heavy camera and long hours, the risk of discomfort is minimized.
I've bought two: the Yeti, which can carry one or two cameras (one on each side of the body), and the Metro, which is the "compact model" for minimal space use when traveling. I am just trying out the Metro (which of course has the smallest shoulder pad), and even with my heaviest camera and heaviest lens, it feels very comfortable. Of course I can't yet vouch for how it will feel carrying it for five hours, but I can say that it does not at all feel like I'm carrying the heaviest gear I have. And that's just with the most compact strap, so that induces trust.
I found out about this strap from a video on Luminous landscape, and it was mentioned that many of the photographers on their long-range camera expedition were using BlackRapid straps, so clearly the word-of-mouth recommendations are strong.
Note: BlackRapid likes to mount the strap via the tripod thread. But if it fits you better (for example to have the tripod thread free), you can also use either side strap mount, though you may have to use a keyring-ring as go-between.
Note 2: Some Amazon reviews complain about the prices, but I think those are people who haven't tried them yet. The Metro I think has a quite fair price ($39). The Yeti costs quite a lot more ($99), but when you pack it out: man, it's impressive. That is solid gear, I think you really get a lot for your money. (Of course it's about equivalent of two good straps, plus extras to secure it around the shoulder, I guess in case you're photographing from a plane doing stunts...)
Also the steel bits are kewl. They seem like they could lift a car (I wouldn't be surprised), and they are even nice to look at, with semi-black anodization (similar to Apple's "Space Grey").