There is a time for low-impact aerobics.

That time: when you’re in a retirement village, killing an hour before Wheel of Fortune comes on.

But there is a time for strapping on a 50-pound rucksack, running in Antarctica or simply swimming in the icy waters of Siberia.

That time: now. Because we’ve assembled some of the toughest workouts on earth to get you motivated.

So, technically, now really is reading time. But tomorrow, you’ll train like a Navy SEAL.