I agree that interval training is the way to go here, but it's definitely more nuanced than only targeting VO2max in zone 4. I'm going to talk from the perspective of cycling. This image is a reproduction of a chart from "Training and Racing with a Power Meter": http://lh4.ggpht.com/-bEo_kR-QvlM/UtSoVtZ1PlI/AAAAAAAAC2s/Q9...
You'll get some benefit across almost the entire spectrum almost no matter where you are focusing your efforts. You only see this diverge at the extremes: too easy, and you're just recovering; too hard, and you're only optimizing the systems that allow you to go really hard.
For some reason when I think of HIIT I picture people as trying to do Tabata intervals and you can see from that chart that you sort of run into a problem: you get the same level of benefit as if you just went slowly for a long time (this part is key -- the chart doesn't highlight how long you should expect to spend in the zone to get the benefit, whereas the training plans in the book make it clear that they expect you to sit in zone 2 for 2-4 hours at a stretch. Compare that to the 3-8 minute VO2max intervals for 3-6 repetitions.), while only really optimizing your ability to endure future Tabata intervals.
The real reason to be concerned with any of this at all is to focus your training and build up the appropriate ability without injuring yourself and without hampering your training down the road. Based on the previous chart, it seems like Z4 and Z5 have the most number of pluses, but they also build up the most damage as they are physically and mentally taxing -- you'll build training plans around 3-20 minute intervals in those zones instead, and fill in the gaps with Z2 work or rest.
If you're time-crunched, you may do abbreviated intervals at Z5, decrease the time you spend at Z2, and shift some of your training effort into Z3 instead. But Z3 starts to get hard enough where it impacts your recovery without giving you the most desirable adaptations. The ideal thing to do here is to increase your training time so you can spend more time in Z2, but we don't all have that luxury, unfortunately.
You'll get some benefit across almost the entire spectrum almost no matter where you are focusing your efforts. You only see this diverge at the extremes: too easy, and you're just recovering; too hard, and you're only optimizing the systems that allow you to go really hard.
For some reason when I think of HIIT I picture people as trying to do Tabata intervals and you can see from that chart that you sort of run into a problem: you get the same level of benefit as if you just went slowly for a long time (this part is key -- the chart doesn't highlight how long you should expect to spend in the zone to get the benefit, whereas the training plans in the book make it clear that they expect you to sit in zone 2 for 2-4 hours at a stretch. Compare that to the 3-8 minute VO2max intervals for 3-6 repetitions.), while only really optimizing your ability to endure future Tabata intervals.
The real reason to be concerned with any of this at all is to focus your training and build up the appropriate ability without injuring yourself and without hampering your training down the road. Based on the previous chart, it seems like Z4 and Z5 have the most number of pluses, but they also build up the most damage as they are physically and mentally taxing -- you'll build training plans around 3-20 minute intervals in those zones instead, and fill in the gaps with Z2 work or rest.
If you're time-crunched, you may do abbreviated intervals at Z5, decrease the time you spend at Z2, and shift some of your training effort into Z3 instead. But Z3 starts to get hard enough where it impacts your recovery without giving you the most desirable adaptations. The ideal thing to do here is to increase your training time so you can spend more time in Z2, but we don't all have that luxury, unfortunately.