The human body is designed for long periods of rest and short bursts of activity, e.g. running away from / after some animal. While sustained exercise does burn a lot of calories and have other benefits, short bursts put muscles into high energy mode constantly so as to be ready.
The human body isn't "designed" for anything but it is remarkably adapted to endurance events. You almost can count on one hand the number of animals that can keep up with a human through a moderate endurance race (5k or 10k) never mind a marathon. Horses, dogs, the ostrich, and a few others. It's no mistake that we domesticated the first two in that list. Humans have even been known to beat the horse at a marathon on particularly hot days.
Point being, I'd question your claim that the human body is "designed" (adapted is really a better word, FYI) for long periods of rest w/short bursts of activity. What you've described there are the big cats and other ambush predators.