Eas pace = pace you are running a5 while being roughly around 140 bpm
I’m all for taking “easy days easy” and “hard days hard” but even on easy days, you need some aerobic stimulus going on. If you are under 120 bpm it won’t be contributing to your pace.
Forget about 50 mile weeks, there is literally no point in that beint your goal right now. Get comfortable with 20 mile weeks. For a 21 minute 5k runner, 9 minute miles are great for 70% of your mileage. Say you were going for a 40 minut run, might find 9 minute miling to be too easy in the first 15 minutes and quite challenging in the last 15 minutes particularly, easy runs should not be challenging but if you are anaerobically WAY stronger than you are aerobically, it is hard to find that balance. I do agree that 10 minute miles are a bit on the slow side in terms of being enough to get proper aerobic stimulus. Sure, do your long runs at that pace but your every day base - I agree that you should start with 2 or 3 miles at 140 bpm/9 minute pace (don’t split hairs and obsess over pace here - if your pace is 9:12 or you are at 136 bpm you are still in the ballpark) and build up to doing this at thesame ffort level for 6 ot 7 miles over 4 or 5 months. Go by feel. I have a great feeling that going by feel for a 20-30 minute run you WILL end up around 9 pace give or take with your current aerobic fitness.
I’m going to stress this again: forget about 50 mile weeks this entire year if injury prevention is your goal (which it should be). Your base is so non-existant right now that you are going to get just as much benefit from 25 mile weeks right now as you would killing yourself for 50 mile weeks.