"Question: are you fresher for workouts if you go long and slow and do not go deeply into the well, or if you go hard and fast, and take longer rest?"
I've asked myself the same question. I'd prefer hammering reps and having a bunch of time off. I did for a while do one hard track interval session; one hard four mile run; and one hard seven or eight mile run; and then just jog two miles on the other days. I did run 17:32 at 53 off that schedule, but couldn't hold form for very long (and I did have 40 years of middle-distance running behind me).
Unfortunately, I think to run a 5000m you have to do enough distance to build some kind of aerobic base. There are a lot of physiological benefits that you can only get by running for an extended period of time.
I think if a 20:00 is close to your true potential you aren't going to get there in a race that is predominantly aerobic without building a distance base. If you are a fast-twitch type, to build up distance without breaking down, you have to run fairly slowly.