Running slowly is important. But the concept of train slow race fast is incorrect. The real key is "train fast at the right time race fast".
I've had some success running, and never is all or anywhere close to 90% of my volume done slowly even when I was up in the triple digits in weekly volume. And even on days where you go slower, it's important to touch on speed like with doing strides.
On a truly easy day I don't worry about speed or pace at all. And when fit my recovery days will be 2+ minutes slower than marathon pace. But my general aerobic pace that I'd do for an unstructured long run will be more like 45 seconds slower than marathon pace. And then obviously my workouts will have paces much faster than marathon pace.
When I was peaking I had a schedule that usually had a structure where 1 days was a general aerobic day with 10-12 miles, a workout the next day, recovery on day 3, another general aerobic day on day 4, a light workouts on day 5, a long run on day 6 and a recovery day on day 7. And strides on 3-4 of those days. I wouldn't hold this schedule while increasing mileage, but this was when I was in the meat of a build. And very little of it is defined as "slow", and this is a very common type of competitive runners schedule (this is a structure that some used on the old Nike Farm Team under Jack Daniels).