Is it possible to improve 5k time by ignoring intervals and focusing on tempos?

Say I was running 4 hours a week (240 minutes) and I spent 20% of that time (48 minutes) at tempo pace. Is that superior to doing 4x mile at 5:45 (23 minutes in total)?

You sort of need both imo, especially as you get under 6 min pace. Once a week, do long intervals at 6:10 - 6:20 pace, 1 min rest for 1200m - 1 mile reps, 3 min rest for 2 mile reps, 4 min rest if doing 3 mile reps. If you are 18 min fitness, this is probably threshold / CV pace so a little faster than tempo. 800m reps closer to 5k pace (5:50 - 6:00) with 2 min rest will help you work on feeling controlled there. Once in a while do something faster, e.g. 12 x 400m at 5:25 pace. You can hit a true tempo pace in the second half of your long run. Other 4 days of the week are easy miles, and one day of rest.

If you feel like you aren't making any gains while including higher intensity workouts, then do a 6 week block where you just focus on threshold during workouts.

Doing the same thing year round works for some people, but not for most. You need to change the stimulus in order for the body to continually adapt.

You'll also get the added benefit of less fatigue, which should make it easier for you to slightly up your overall mileage.

Yes, it's possible but that doesn't mean it will happen for you. I knew a guy who was running mid fifteens in his mid 40s who never did anything faster than nine minute pace, nothing else, no reps, no tempos. He did race fairly frequently most of the time but not always.

Problem is you don't improve running economy at faster phases than you run during training so you need intervals for this distance. Ps I hate them

You probably could, but it's not the most efficient method.

It also depends how maxed out you are. There is a point, for optimum performance, you are going to need a period where you focus on intensity and race pace.

Check out the Norwegian singles approach thread. It's more or less what you're suggesting and some people seem to be having success with it

Yes

I improved my 8k from 25:30 to 23:45 doing just tempos. If you do them right (ie not too fast) they work. There is no real formula for the right pace, it is about feel. You should feel like you are under control and not struggling to hold pace. Your first mile should not be the same pace as the middle miles = ease into it. This is not a race or time trial. Overall time is meaningless.

Do not try to better your previous tempo time from one workout to the next.

just find the pace that your body gives you on that day and run there for a while.

How long or how far? Let your body decide.

when I was in 23:45 shape (4:45+) race pace, many of my tempos were in the 5:20- 5:40 pace. sometimes faster (as low as 5:10 for 6 miles) but often 45 slower than racing pace for about 6 - 10 miles. first mile was almost always around 6:00 pace. I actually didn't even run on the easy days = so I ran 3-4 tempo per week.

no need to do 3 mile stuff for a 5k