Mui thai is rough, its a high damage sport like boxing but worse cause it brings in kicks, knees, and elbows. They do however in Mui Thai emphasize fitness as its a competitive sport with multiple rounds so fitness is key, especially if you ever want to compete.
Japanese Jui Jitsu is lots of grappling and throwing and locks. Its basis is to restrain or move the opponent away rather than hurt him much. To be honest if you want to go the Jui Jutsu route rather just try Aikido as it does that better with even less violence involved.
Other martial arts that dramatically affect fitness are probably more your Chinese ones. I have attended or observed classes of a wide range of martial arts and the Chinese Kung fu was the most physically demanding due to the incredibly low stance you maintain for most of the session. It really works your legs.
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