I eat peanut butter. By the spoonful. Probably 6-10 spoonfuls a day, sometimes more. Awesome.
Now to the five minute rule. It's more of a goal or a benchmark really - on my 20 minute workouts, particularly with biking and running, I usually try to be pretty worked by the five minute mark. As of late i've consistently been doing one minute work and rest intervals, following a two minute 'warm up' which is more of just a steady pace designed to let my heart rate climb. Five minutes in i've completed two work intervals. A week ago during my run i outdid myself and was completely spent after the first interval - there's nothing like really high intensity intervals to make 20 minutes seem like a lifetime. But this is the point really - how much can you get out of that time? The biggest concern that i can think of for a reasonably fit person is risk of injury by hitting it hard right away. I'm somewhat sensitive to this, particularly with running. going from an 8 mph pace to 12+ mph after 2 minutes might not be a good idea for everyone, but there are easy ways around this, particularly if you're inside on a treadmill. I've been running at incline lately - i can go from a 3 mph walk at 15% incline to an 8mph desperate run without really risking as much physical injury. Its my lungs and Heart rate that make the effort seem so brutal.
But what about that spiking heart rate - shouldn't that also be a concern? There may be those that disagree with me, but i don't think so - at least not for those who are in good shape to begin with. After all, one of the physical benefits of good fitness is a responsive heart rate. It goes up fast and it comes down fast. Good 400 meter sprinters, for example, go from a resting (or slightly elevated due to anxiety/anticipation) heart rate to pretty near their max HR in under a minute every time they race.
The nice thing about being significantly physically challenged 5 minutes into a 20 minute workout is that even this briefest of weekly workouts provides a opportunity to also work on cultivating the mental willpower/toughness that is so crucial to having success in serious endurance sports using a limited volume training approach.