Sundew, looking again at your post, I agree with the poster who said that the balance of your training is off. I think your hard workouts are wiping you out. Maybe it would be helpful if I share one key to my own training over the years, which held true into my 61st year. I've always run 6 or 7 days a week, but M/Tu/Th/Sa have always been 30-40 minutes (3 or 4 miles) at a very easy pace--the Van Aaken pace or Maffetone pace, you might say. A slow jog. As I've aged, that slow pace has slowed; right now, on the comeback trail, it ranges from as slow as 11:45 pace to as fast as 11:00 pace. I use a HR monitor to keep it down. But simply monitoring my breath and making sure that I can breathe at a 3/3 rhythm with zero trouble, zero breathlessness of any sort at any point in the first half mile--all that keeps me on track.
It sounds as though your 4 days a week schedule cuts out these filler runs. I can see why a 60 yo would do this: less wear and tear. But the downside of that is that much more of your running is somewhat harder. Walking is OK, but it's at a HR lower than these very easy runs. It's more like 50-60 % of max HR (in my experience), not 70-75%.
So that would be my first recommendation: learn how to back off the throttle and make these slow jogs a part of your schedule.
I also think that you can dispense with formal interval work, at least for the bulk of this next training cycle, and get your threshold and speed work with progression runs--slowly notching things up towards 90% in the last 2-3 miles of an 8 mle run.
In place of 400s on a track, I'm fond of a modified tempo run that I call tempo/intervals. It's a sort of Mono fartlek, named after Steve Moneghetti.
I do mine on the "back" leg of an out and back run on a flat country highway.
I set my watch to alarm at .25 miles. After a 3-5 mile warmup with a few strides towards the end, I start with .25 pretty hard but controlled. Then I slow, as much as needed, to an easy pace, letting heart rate drop until it reaches tempo pace, then continue at that pace until the second .25 beep goes off. Then another hard .25. Then another easy-to-tempo .25.
That's the first mile.
Repeat alternating quarters for the second mile.
In the third mile, I begin the same way--hard quarter, easy to threhold quarter. Then, in the final half mile, I aim for a pace somewhere between hard and threshold. It's 5K/10K pace, more or less. Racing pace, but not full-on hard. And I do my best to hold it for the entire last half mile, ignoring the beep at 2.75 and pushing on through.
This has some of the effect of a 3 mile continuous tempo run, but with an additional stimulus. It's great practice for hilly races where HR will vary through a range of harder paces. You can adjust how much you slow in the easy-to-threshold pace; if you hit things TOO hard during those sort-of-intervals, you'll know, because you'll have to slow way down.
Enough from me. Good luck with your journey! There's much to discover here.