Researchers wonder if 6 minutes of hardcore interval training- extremely intense 2 minute bouts of very hard exercise- might build fitness as well as hours of comfortably-paced exercise:
Six minutes or so a week of hard exercise (plus the time spent warming up, cooling down, and resting between the bouts of intense work) had proven to be as good as multiple hours of working out for achieving fitness. The short, intense workouts aided in weight loss, too.
There’s a catch, though. Those six minutes, if they’re to be effective, must hurt. “We describe it as an ‘all-out’ effort,”… You’ll be straying “well out of your comfort zone.”
Article, with info about how to try this, here.