How to and how long to break 18 minute 5k?

not a chance

Right now I'm doing 25-30 mpw with some tempo, and 1 long run (8-10 on Saturday), with a break on Sunday.

My pr is 18:33 and I'm also trying to get down to the mid-18s over time. Right now I'd probably say I'm low 20s.

You don't need to run anywhere near 80 miles a week to run sub 18. You don't have to run 50 miles per week either. Do a 3-5 mile tempo run each week around 6:20 pace, do an interval session and do a longish run with a pickup.

You won't need 80 mpw as you have run a 4:40 mile. 40-50 will do it

Once you start introducing reps / hills tempo your times will start coming down further. I would not wait too long to do this. 6 weeks steady running should set you up

Plus weight is a big factor.

Back off if you feel niggles coming on.

You can definitely run sub 18 in time. Be patient and just go for pbs although you can hit a plateau then times just fall off in one go.

it's common sense that running on grass and firmish sand will be better for your legs because it's softer (but not too soft, you can go too soft and sink into the surface which is bad). same like running on a track is better than running on concrete.

similar sort of thing as running 5k barefoot and running 5k in shoes with a lot of cushioning. barefoot will put more strain on your muscles and bones.

off topic a little, but i run barefoot road races up to 21k. and sometimes run in normal shoes too. when i run barefoot the muscles are sore for a couple of days afterwards, and generally when in shoes the muscles aren't. i think it is beneficial in a way to run barefoot like this because you are breaking the muscle down so it rebuilds stronger, and you have to do less work to get to that stage (less mileage) than you would when in shoes. you might have to run 42k to put the same load on the calf muscles when wearing shoes, but only 10k when barefoot. but there is probably a line where you are doing more damage than is healthy or where you are losing too many training days to DOMS.