Extensive intervals, are hard no doubt but look at it like this... After you pass a guy in a race do you get 3 minutes to recover before you attempt to pass the next guy? Plus we all do not recover the same and the ability to push on is your ability to go again.The intensity is very high and I wouldn't let a newbie train in these zones right off the bat .It is one where a person has built the aerobic base gone through a lot of high endurance ,Intensive Interval, power training at LT and knows basically what it takes to be at the edge of breathless while performing high effort work. This type of work will be what it takes for you to become more fit by overloading an all ready fit condition.
Velocity