101: Physical Fitness. Even if you have all the food, guns, ammo, and other supplies you need to last years....if you are 350 lbs. and break a sweat opening a beer, you aren’t prepared.
Caveat: Those pounds will melt away in the weeks following SHTF, but one still should be prepared in advance, of course. Also, if you're not in good enough condition you may not last for weeks.