Training & Nutrition - benefits of zone 5 training

I'm pretty new to zone 5 intervals. I've been working my way up and can hold zone 4 for about 2 hours, but lately I've been trying to get in some zone 5 work and it's hard! Thankfully, the more zone 5 work I do the easier zone 4 feels (duh!). This got me thinking, besides the physiological benefits of training in zone 5, does anyone feel a psychological benefit of zone 5 training (i.e., dealing with the pain). What about other benefits? Does the human body have "limiters" that prevent us from over exerting/injuring our selves. Are these limits raised by interval training? Thoughts?

Define Zone 4, because my Zone 4 is threshold which I certainly couldn't hold for 2 hours.

I'm using Joe Friel's zones. Zone 4 is below Lactate Threshold (LT), ranging from about 94% to 99% of LT heart rate. Zone 5 ranges from 100% LT heart rate to max heart rate.

Best thing I've done this season for making me fast was to find a fast group ride and drive the pace for 20 miles just as hard as I could. I've done a couple of times where my HR stayed above 180 for 10 miles or so. My max HR is 199.

Get a paceline set up, pull hard for a couple of minutes then rotate off. Drop 3 riders back or so and repeat. You need enough recovery time to take the edge off, but you want to keep your HR high.

I should mention that you should have a HR monitor so you can be sure you don't completely blow up, and that you need to have a good feeling for how hard is too hard - you don't want to get dropped 5 miles later. The pain is exquisite. You'll know it's time to stop when no one wants to pull through after you! :)

So yes, spending time in zone 5 can make you faster. Yes, it is going to hurt a lot. A whole, whole lot. If I remember correctly that training plan prescribes very little time in zone 5. There are other training methods out there that include more time in zone 5, so feel free to experiment.