I suppose the question everyone wonders about with Kawauchi is how fast he could run if he only did 3-4 marathons a year. In interviews I've seen he doesn't seem to think he'd go faster or maybe even as fast as he does now with all the races. I recall an interview with Stahl where he said that he thought that because he couldn't run as fast as the absolute best guys he didn't do as much damage to his muscles as those guys did and therefore could recover faster and race more. Doug Kurtis had a best of 2:13 (?) and I know he once said he'd rather run ten sub 2:20s in a year than one 2:11. Again, I may be a minute off on his times. But maybe Kawauchi is a potential 2:06 guy, at least physically, who's just not damaging his muscles much with a ton of 2:08-2:12s.
I know that Kawauchi pays a lot of attention to recovery, hot baths, cold baths, acupuncture, etc. and I imagine those things play a role. But I suspect that the real key is that he WANTS to run all those races. We don't talk much here about the mental side of running. I imagine that's because it's very hard to quantify. But I suspect that his big "secret" is that he likes doing it so rather than look at reasons not to run marathons he looks for reasons to do it. But that's all supposition.