(Coggan zones)
Power Training Levels | TrainingPeaks
Where a traditional interval session is intense but short doing intervals spread throughout an endurance ride is more of a slow burner that grinds you down towards the end. They're both hard but in their own ways. The main reason for doing a 10 minute level 2 period between each interval is to keep the amount of level 5 to a vaguely manageable level. 1:1 between 5 minute intervals and level 2 periods for a four hour ride would ramp the difficulty up massively!
If you're doing a four hour (240 minute) ride, beginning with a 15 minute (level 2) warmup and then 5 minute intervals (level 5) with 10 minutes endurance pace (level 2) in between for the entire ride that breaks down to 75 minutes level 5 and 165 minutes level 2. That's a
lot of level 5 work. A 10 minute period between each five minute interval gives you a chance to get deeper into the four hours before the fatigue really starts to build up. That's your normal level 2 endurance pace too, not easy level 1 recovery spinning.
The idea behind doing something like this is that you're doing an endurance ride interspersed with frequently lifting the pace. The 5 minute intervals are equally spaced throughout the ride, so that you're doing hard efforts in hour 1, hour 2, hour 3 and hour 4, and you're doing them in points you wouldn't normally. If you do a standard interval session and then tag on a few hours of endurance work afterwards (something I do regularly as well) you're getting a short sharp first hour but it doesn't really get you ready for continuing to deliver hard efforts in hours 2, 3 or 4. If anything that format (intervals first then several hours of endurance work) reinforces the idea that after the first hour you're going to have to ease off, which isn't the case. It's all about being prepared in advance for whatever may happen, so that you're not intimidated by the prospect of going outside your comfort zone at any point in the race.
Intervals throughout an endurance ride also has an important psychological benefit. When you're riding alongside another rider several hours in, they're hurting, you're hurting but you
know (because you've done it already ! ) that you're capable of doing 15 hard 5 minute bursts and they're going to crack first.