Earning an obscene amount from boxing that gives you and your family the quality of life others, hard working people, could only dream of should be more than enough motivation. I'm fed up with hearing boxers saying that they have no motivation, if that's th case quit and get a job.
Fury doesn't even look in that great shape tbh. Yeah he clearly has 12 round stamina but he's still carrying far more body fat than he did before the layoff. He's had enough time between the wilder fight and now to get back into that shape, his diet must be horrendous, especially if he's training harder and more consistently now(like he claims) He was fatter Vs Schwarz than wilder Billy can make 160, but he needs the right motivation to do so. No point in cutting down that far for a bumfight
Fury never looks in great shape. He was flabby around the mid section against Klitschko. That's his body no matter what he does. He was heavier v Schwartz than he was against Wilder yet he was visibly faster because he was in shape for a long time. I rewatched the Wilder fight the other night and Fury's fitness is notably exceptional. He looked fresher than Wilder who is a fantastic athlete. I've seen plenty of clips of his recent training and he's running 7/8 miles every other day with fighters half his size. I don't think Fury's weight and appearance is of great importance. What's more important for him is training and activity. With the level of fitness he brings to the ring, I've no doubt his camps are elite from training to rest to nutrition.
Fury likes a beer so probably doesn't help with the weight but he has Ricky Hatton on board who is an expert at the balloon method. I know Peter was big on old school roadwork too so that's probably helped his stamina he always trains his fighters for 12 rounds.