Having a hard time “Running Slow”

I’m a relatively new runner, I’ve run in the past but only recently really gotten back into it. I’m averaging 25-30 miles a week, but struggle running at a slow, “loping” pace. On my fast days I can run at a pretty consistent 7:35-7:50/mi pace for around 5 miles. My “slow” days are where the problem comes. I have trouble doing anything slower than 8:30/mi without feeling like I am being unproductive. My most recent one hour run was a 7.25 miler.You have to run slow to run fast... so how do I focus less on running quickly, and more on building up my endurance for half and full marathons?