I'm guessing you're meant to do the exercises one after another with no rest? I might give this a go when I'm 100% again. I know I could probably do the situps and pushups, 20 strict form pullups would kill me though.
i don't think it's quite with no rest. it was meant to be done with large groups, in waves. you can probably take 5-10 minutes between.
also, are you sure about the sit ups? they're to be done with legs straight, and elbow to opposite knee at the top of the movement. takes a bit longer than a regular sit up. i don't think i could get 79 in 120 seconds. or 54 push ups chest-to-floor, for that matter.