Players in Japan are lasting longer than US players. And they are throwing and playing harder. Is it the training here or how they are brought up as kids.
Plus they get a lot more rest during a 162 game season then a NFL player gets during a 16 game season.
Now add in spring training and post season and you are playing 8-9 months.
Baseball goes at a slower pace. But the players are arguably getting to fit and built up to play.
There are a few differences between MLB and pro ball in Japan. I believe they play about 140 games / season and they definitely have much less travel. Also, don't they have 6 man pitching rotations?