Kane's always been in good shape and while he's always had pretty decent definition in his abs, he's also had a bit of a belly. I noticed on SD! last night that his stomach's flatter than I've seen it in years and that he seems to have a little more general tone in his upper body. Considering that he turns 47 in about 6 weeks or so, he's in phenomenal shape, better than most men half his age.
I can't say that I'm all that surprised, however, when you consider that Kane's no longer as remotely active as a wrestler as he used to be. As the "Director of Operations", Kane hasn't been wrestling as much as he used to. As a result, he's got more time in-between to rest, get as much sleep as he needs, can devote more time to hitting the gym, doesn't have to deal with the various aches & pains associated with being in the ring nearly as much and can more easily maintain his diet. Altering and maintaining a more nutritious diet is a huge factor and probably has more to do with it than anything else, as he's able to get the right kinds of foods and have them prepared in the right kind of way.
About 10 years or so ago, Triple H was a little chunky himself. He started putting on muscle mass in the late 90s or so because he had to be a good 30 pounds or more lighter when he was doing his "Connecticut Blueblood" character. By about 2000 and into 2002, Trips was friggin' ripped and had to be a legit 250-260 pounds. However, when he lost a lot of cut in the coming years and I think a lot of that had to do with the physical demands of the road. Trips was one of the top guys back then, which meant he had to put in an extra workload. Sometimes, I'm sure that meant that he was so tired and sore that he didn't feel like hitting the gym nearly as hard as he once did, nor did he have the luxury of eating properly all the time. However, over the past several years, Trips is back to a much more lean build, which is FAR easier for him to do these days since he only wrestles a very small handful of times a year now.