You'll need more food and water than you think.

[MaryN]

Well, if you know that 1200 calories isn't going to be enough for such a strenuous exercise, it does seem a bit pointless to do it just for the sake of finding out if you can. The most strenuous armed forces exercises (probably those in Norway and cold climates) provide dried food which are far, far in excess of 1200 calories, and those guys really know what they are doing. Does it seem likely that in a SHTF scenario you will be lifting weights, biking 10k and doing all sorts of other dodgy things? I suspect most people will be tucked into their homes or bug outs and conserving energy.