It doesn’t look like I’ll be able to make as much time as I’d like to give this topic a proper post, but I did want to start with something vs. waiting for that day to arrive. A bunch of this started much earlier, but was jiggered in my mind via this post - http://www.diabetesforums.com/forum/topic/77325-finally-decided-to-read-bernstein-with-reservations/?p=939297 - which, in a nutshell, called out higher intensity training as the best way to go on the exercise front.
As I mentioned then, I found the article very well written and great advice in some contexts, but strongly believe it’s not the best advice in other contexts, in particular sustainability in those without the equivalent willpower  of Dr. Bernstein, or physical capability to safely engage in higher intensity, more efficient physical activity.
To be clear, there’s almost no question in my mind based off personal experience, exposure, and research over decades that higher intensity, “purpose training”  obtains superior results, especially per unit time. And by results, I mean the same class of outcomes as what Dr. B observes - in his case, running further, faster, having more of what he calls stamina and capacity. So the best bang for the buck in many ways.
But if your goal is sustainability, and not necessarily this efficiency (results per unit time, best use of our busy schedules, etc.), consider some of the downsides of prolonged high intensity training:
- Ability to maintain high intensity over truly long periods of time. Five years would be the absolute, obscene minimum I’d consider “long term” - it really should be measured in decades.
- Correlation of intensity and injury, even for those with perfect form, knowledge, experience, and so on. Ironically, I see analogies with this correlation and a variant of The Law of Small Numbers theme in his book. Big intensity, bigger potential for things to go much “more wrong more quickly” before you realize it (and “right more quickly” to be fair). =) I'm seeing a TON of this with the new Cross Fit generation at my gym. Tons.
- Even outside my domain of fitness and athletics, I think there’s something to be said about the longest lived populations on the planet and some of the things they have in common. None of them train in this way. Certainly nothing remotely similar to HIIT or variants. But all of them maintain a relatively high level of daily, regular, activity. Exercise or activity vs. purpose training with no analogous goals (speed, strength, stamina, capacity, etc.). 
- Lest you think I’m not a fan of efficiency or even HIIT. Quite the contrary. I think it has unbeatable usefulness in many contexts and even some analogies to periodic or 5:2 or abc fasting, jump starting xyz, and so on. I.e. where the ratio of the activity vs. the non-activity and the benefits is extremely compelling. Incredibly useful and almost irreplaceable.
I just think there’s an alternate motivation in play here, another viable option to integrate fitness into one's lifestyle, and that there are benefits beyond the great ones associated mostly with higher intensity routines.
In my n=1 situation, I keep intensity as one of many tools in my toolbox. And it's not an on/off situation either, there is obviously a spectrum of intensity. In general and to date however, I've gone down a far more mellow but NEAT  oriented route. I enjoy regular walking and using some of that time to catch up on podcasts, news, shows, learn or hear about new things, and so on. And just unwind. And while diet is my number one "go to" tool, there are certainly many benefits to physical activity that go beyond or don't even involve blood sugar levels. For some reason, I've been hitting more towards 100,000 steps per week vs. my baseline minimum which was around 88,000. Not intentional, just happens, I don't notice it.
And sorry about the vomit in the entry title - just a catch line given how many folks have clearly never done high intensity training or are surprised that vomiting absolutely, definitely comes with the territory. While most of this is friendly joking - you can obviously do intense anaerobic exercise without vomiting - without exception, 100% of all competitive athletes I've known over the decades experience what I'm talking about with regard to vomit and intensity. If you've never pushed this hard and crossed that line, you have NOT come even remotely close to your limits in terms of anaerobic intensity. As always the inverse or the converse are not true - you can certainly vomit and that does not mean you're exercising with intensity. Many just spend time hanging out in the aerobic zone and use a heart rate monitor to measure intensity. =)
And to be clear:
- This vomiting is not tied to the GI distress so common with distance running. Totally different root causes.
- You also don't have to vomit every time you engage in high intensity exercise. Or even most of the time as you become more familiar, know your body a bit better, and gain better fine control and awareness over your routines, how you feel a given day, etc. But if you do hang out in the highest intensity zones, it's just a matter of time....
- My current take and use (or relative lack) of intensity in my routines has nothing to do with vomiting or avoiding vomiting. It has to do with sustainability, my own observations, and my own perceived results. No scientific rigor is involved. At all.
 Or whatever you want to call it - I don't mean to be derogatory, merely pragmatic for the general population.
 This is another huge topic and was somewhat controversial decades ago. In short, it’s tied to training specifically for an event, season, and competing. There are elements of time, both in terms of how much time is available to prepare, along with peaking with regard to peak performance for a given schedule of events, tournament, or related scenarios. There’s also an element of specificity
 Non-exercise activity thermogenesis (NEAT) is the energy expended for everything we do that is not sleeping, eating or sports-like exercise.