I always took Karkull's energy to be like radiation, so that when he exploded, he irradiated most of the East Coast and possibly parts of the midwest with his radiation that drifted due to a North Westerly breeze. That way, many people in the US look much younger than they actually are due to Karkull's radiation, but the JSA took the brunt of it, so they're the ones who benefit the most from the radiation.
Hmmm. I like this one. I'd modify it a bit so that the surrounding countryside got only a tiny dose but in keeping with the radiation poisoning analogy the JSAers severely "contaminated" those they came into contact with frequently, so the wives, girlfriends, sidekicks and friends, etc. received the longevity benefit as well.