People who run very slow easy days (10:30 or slower) what works best?

Good question. I hope you get some responses.

I figured out long ago that I needed to divide my weekly runs into three longer, harder runs and four short slow runs. In practice, that meant W/F was 6-10 miles, Sun was a long run, and M/Tu/Th/Sat was 3 miles very slow. Over the years, as a masters runner, those 3 mile runs have slowed from 9:30 pace to 11:45-12:00 pace. The W/Fr runs have slowed from 8:30 pace to 9:30-10:45 pace. The Sunday long runs have gone from 15 miles at sub-9:00 pace to 10 miles at 9:30-10:30 pace.

The key to getting your short easy runs paced properly is tuning in to what a truly relaxed pace is. Knowing your actual max HR is important, but listening to your body and your breathing is always the bottom line. Do NOT run all your runs at the same effort. Trust the light easy pace. Read Ernst van Aaken's "Van Aaken Method" and understand the power you generate when you run aerobically.

When I was running 20 minute 5Ks as a 50+ (6:25 pace or thereabouts), I was running those short runs 3 to 3 1/2 min/mile slower.

I'm 68. Three years ago I ran 7:37s for a 5K and my slow/recovery jogs were 10:30-11:00. I've never been afraid of that differential.

Always start slowly. Only a fool starts an easy run harder than very, perceptibly easy.