Anyway, it's an even bet we're about the same vintage, so I'm reluctant to ascribe the problem to old ageBeven though there may be some truth to that. Could be your hip cartilage isn't as elastic as it once was, so that causes some soreness and stiffness. Even could be very nascent arthritis.
So what to do? I will say that the Therm-a-Rest GuideLite ($75; www.cascadedesigns.com), while a pretty good mattress, is not the plushest thing on the face of the earth. It's the same size and thickness as the "standard" Therm-a-Rest, but uses foam that has been die-cut to save weight. That also means there's less foam for cushioning. So you might want to add a little padding to it. One good way to do that: Throw a Cascade Designs three-quarter-length Z-Rest ($30) over the GuideLite. The Z-Rest adds only 11 ounces to the load, but it will add a LOT of cushioning over an inflatable pad, plus some extra thermal insulation. That alone might solve the problem.
Alternatively, just try a thicker pad, at least if weight is not a real issue. Cascade Designs' LE MegaRest ($200) is about the thickest inflatable pad around. But, it also weighs five pounds. Slumberjack's Family Camper is lighter and less expensive ($90; www.slumberjack.com) and almost as cushy.