I went through the whole pre-op training program. I have mixed feelings about it. It annoys me that it cost me a month and quite a bit more money to do, but I think I can see how it may help recovery (though, like MDoW said, the month you have to train kinda cancels that out anyway). My legs were pretty strong, but the Cybex test revealed that I actually needed more improvement than I thought. I took it very seriously, and really increased my leg strength and lowered my total percent body fat even more to like 11% or something (wonder what it is now...lol). Before LL, even though I was a gym rat, I never worked out my legs much because they were naturally muscular (I focused on arms/chest for aesthetics, and just glutes for my legs). My legs were, in turn, deceivingly weak.
I think the concept is that, all doctors agree we quickly lose tons of muscle mass after LL (since we're just mostly sitting around). Dr G I think is of the opinion that, if you can increase your muscle mass to, say, 130% of what you used to be, and then you lose 60%, you'll be at 70% of your ORIGINAL capacity, as opposed to just 40% of your original capacity. And this can help with weight-bearing after you're done lengthening. I think it's helping me now, even though I still have pain (but that's mostly from tightness and stretching the muscles and such).