Respect everyone's opinion, but personally I think most of this is old-school nonsense. This is coming from someone who considers himself old school. I fell into the trap of falling for a lot of this myself for many years.
If you only have hobby hours, the vast majority of people would get faster just by sticking to NSM instead of worrying about anything race-specific. We already have a huge number of people saying the same thing about the 5K—and even shorter distances.
I’m way faster in the mile and 5K now than I was years ago, and I’ve more than doubled my age. I’m still only running 5.5–6 hours a week, spamming sub-threshold work. I’m just fitter overall.
No hobby jogger is ever going to reach 100 % of their potential in any event. It’s laughable to even think that. So you might as well fill the fitness tank as much as possible. Spamming sub-threshold is boring, but it’s the easiest way to do it.
I used to do one threshold session plus some 5K-and-below specific work and still only ran 5–6 hours a week. Here are the two factors that changed everything for me:
Spamming just sub-threshold lets me stay far more consistent—which is a massive benefit.
Three sub-threshold sessions are worth more than one 25–30 minute threshold session plus 15–20 minutes of faster-pace work. Ninety minutes of sub-threshold work week after week will simply make you fitter. You’ll run faster. It’s not rocket science—even at 5K and below. I bet I’m not the only one who wishes they had known this when college coaches told us you have to “practice running fast to run fast” even in the scraps of the depths of D3. Looking back, it was ridiculous. I would have had far better results just doing NSM (or any equivalent base training) year-round and getting into really good overall shape.
The big caveat, of course, is the elite level. There, everyone is already almost as fit as they can get, so specificity becomes the final difference-maker. But to think the same rules apply to hobby runners is just plain stupid, in my opinion. I say this because you mentioned “to run your best 1500.” Again, almost nobody here is ever going to run their absolute best 1500. It’s about getting as fit as possible so you can run as close to your potential as you can. Nobody is anywhere near their potential. Climbing as high up the fitness chain as possible is what gets you closest—not specificity. People still cling to this idea blindly because it was drilled into them by dinosaur coaches who only really understood elite training. Even at the elite level, plenty of top athletes are now challenging how much specificity is actually required.
This is probably the biggest myth busted by the whole conversation in the thread . At the very least, it has clearly shown that specificity is wildly over-inflated for shorter distances—and it’s probably not something most of us need to worry about at all. Or at least until you are certain in the medium term you can't improve via an easier way.