I always like it when stories get exercise advice right, or at least, mostly right. Taking rests is super important, so bonus points there, manga usually fall prey to the "more training = more better," blunder.
Low calorie diet... eh, it's situational advice. If you're trying to lose weight, yeah, do that. If you're trying to build strength though, not a chance. You can easily burn 500 calories in an hour of aerobic exercise. You need those calories, the law of conservation of energy will not be denied.
(Also, holy buckets! She told someone she reincarnated before? Is anyone ever going to figure it out? )