Why can I run 18:30 off 20-30 miles a week of slow jogging, but sub 18 requires lots of hard workouts

Well that's 6-12 seconds per km faster, which is significant. I can run 3.19 per km for 10k, but I cant run 3.13 per km for 10k - it's beyond my current ability. So I have to train smarter/harder and get faster.

Base and Peak shape.

If you would do a base with 50 miles of jogging and steady you could probably do sub 18 of it and then run a new pb once you do a block of workouts

Also, I would try looking for the flatest course possible.

3 weeks of 2mi warm, 10x400 @ 74-76 2 mi cool before.

You should be good

The funny thing is I would struggle to run 75 seconds. My best 5k was 17:32.

Let me give you some context. 1830 to 19 is probably your tempo pace. When I was at my best I was roughly 16:00 to 16:15 for every 5K I ran with long runs, speed work and tempo runs. When u just jog u never get out of your confort zone. Speed work teaches you to run at or faster than race pace. If I just jogged back then I probably would have run 1630 to 1640 instead of 1600 to 1610. Speedwork help endurance and teaches u to run faster and to hold it for longer periods of time if done properly

There's your problem. Always thinking training has to be hard. It shouldn't be.

I'd call it "base" (baseline) vs training fitness

I think it's very easy to get back into ~10s / mile off PR shape (if you take 1 week off). But it takes much more to get back into PR shape.

I've been trying to change my approach by doing many slower workouts like 2 mile repeats at 6:25 (4:00 per km) but no success so far.

Because 18:00 is faster than 18:30.

Still all wrong. Slower isn't the answer. TWO MILE REPEATS? Who filled your head with so much junk? You need to be shorter/faster stuff, including hills. 150-600m. No long runs, no mile repeats.

What minimum mileage with workouts do you suggest for breaking 19 minutes based on your experience? Thanks

The obvious answer is that you can to a level of fitness with just a bit of easy running but it will only take you so far. That said, there are other ways to get fitter. Running more than 20-30 a week will get you fitter. I know a guy who has gone much faster than sub 18 when he was in his mid forties and did nothing but very slow runs but more than you're doing. I do believe you could race yourself into faster shape if you raced enough and did it at both longer and shorter distances.

Specifically to your question, I don't know what you consider a hard workout, but your muscles need to adapt to running at a sub six minute pace if you're going to do that for any significant distance so you'll need to do something at that sort of effort to get them to do that. But it doesn't need to be all that hard, again, saying that I don't know what you consider hard.

I think its a matter of consistency over a period of long time (months/years).

If you continued doing your 20-30 miles perhaps with strides at the end of most runs or at least surges within runs n maybe increased your mileage to say 30-35 miles l would say you would run sub 18.

600m hill repeats once a week so you are breathing really hard at the top walk down

one day of 150m repeats jog

one day 3 x 1 mile at 550 to 610 pace

between the above, other days cross country at a slow to medium pace if you feel good.

take a day off

It likely won't make a difference if you're still only running 20-30 mpw.

Doing threshold workouts is a good start, but you need to up your mileage somewhat.

We've all got talents and skills.

I'm not too old yet, but I can be out of shape and overweight and I can run a ~55s 400m off of just some easy jogging and strides. It feels natural. A few weeks of training and it's a 52. Anything faster than that takes a lot lot lot more training.