You're getting a lot of strength and endurance answers. Both are important to being in "dive shape."
Flexibility is also very important - you can be strong as an ox, or have the endurance of a marathon runner - but if you can't reach back to your valve and turn it back on you're going to have a very bad day if the absent minded DM turns it the wrong way.
Same with being able to reach behind you and find that piece of mono that's hooked on the cut in your tank boot, or reaching down to grab the kelp off your fin strap. Or if you're a photographer, being able to contort yourself into a crack to get that killer macro, or quickly follow the playful sealion and compose the shot. Or holding an arched back throughout a 70 minute dive to keep your trim good and the silt clouds to a minimum as you hover 4" off the bottom.
I do a lot of cardio / endurance training (5 - 7 times a week) and a reasonable amount of strength training (2 - 4 times a week) - being able to do long swims is important, and so is using less gas. Being able to don and doff in a pitching sea or climb out of the surf is important. But having the flexibility to manage small emergencies and annoyances has proven to be valuable, too.
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Ken