I'm torn with my thoughts on Jim.
Don't know the guy at all other than what I see his training (and results) are like, as well as some interviews. Dude just seems like he loves to run. Realizes he might not be able to do this forever, so why not just do it while he can. More of a "live for today because tomorrow is uncertain" type runner. I respect that because it's true. Maximize what talent you have.
At the same time, given his recent history in the 100 mile distance, I can't help but think of another "prodigy", Anton Krupicka. Same type of runner. Logged HUGE weekly mileage with elevation in the mountains. A few really good race results but then got hurt. Coming back from injury it was back to the same routine of huge weekly mileage and elevation. Got hurt again. Comes back, huge miles again. Hurt again.
What makes Walmsley "great" is the hard, mega-miles/elevation in training. What made Krupicka "great" was the hard, mega miles/elevation training. What caused his downfall was the hard, mega miles/elevation training.