Intervals are the hardest workout I've done, and the second most painful experience.

The most painful experience was from a busted up back and neck from a car wreck. The difference is like the difference between horseradish hot and habanero pepper hot. One is painfully hot for a few seconds and then gone almost as quickly with no lingering effects. The other is painful, doesn't let up anytime soon and hurts almost as much the next day.

It's really helped my recovery time, especially after hill climbs. My rest periods are much briefer -- usually I can recover while riding, just by easing up on the effort -- and I'm ready to go again, sometimes stronger after a warmup ride of 30-60 minutes.

I wish I'd known about interval training when I was an amateur boxer. Back then we thought going all out for 3 minutes, followed by a minute's rest, was the best method. But we should have included more intense bursts of 10-30 seconds within those 3 minutes. I was often gassed by the third round because we didn't train for bursts of effort and relative rest within the 3 minute rounds.

Interestingly, a form of interval training was what helped prolong the careers of Muhammad Ali, Bernard Hopkins and Floyd Mayweather. All were masters of stealing rounds with bursts of effort and flurries, followed by defensive tactics to rest and frustrate opponents. Something to keep in mind for the 50+ body.