As you may know, Arthur did not consider interval training to be "speed" work. He was referring to short work which maintained or increased leg speed. The classic thing to do was "fast, relaxed, strides" of 100 meters or so with FULL recovery. The other thing he mentioned was doing light hill work possibly as part of your second run. This might be something along the lines of 15-20 minutes spent running up and down a hill fairly easily as opposed to the formal and more demanding hillwork done in the actual hill phase. This would be done during the base phase and the imprtant thing from his perspective is to avoid going into oxygen debt while you do it.
Niether Arthur nor Barry have EVER advised me to do any particular pace or effort level while doing basework. Arthur's classic advice was, "Enjoy yourself." Thet never really satisfies very many people but I think it's the best answer.
For those wanting something a little more specific I'll recount two things I picked up in conversations from Snell and Magee.
Peter told me that during the early part of the base phase the weekend long runs were usually done at about a seven minute pace but as the phase went on and fitness improved it would drop to something close to six minute pace. On shorter runs, the 10-12 milers, he said they'd go "a bit" faster. It would have been on those shorter runs in the later stages of the base phase that you'd have seen those 54-55 minute 10 mile runs. But again, you need to rememeber what those guys were capable of doing when they ran at full effort. For guys like Baillie, Puckett, and Magee those ten milers were slower than their marathon pace.
The key, Peter said, was to be able to follow the schedule day in and out. If you ran too hard, he said, you'd struggle over the next couple of days, perhaps needing to run ungodly slowly to complete the distance or perhaps needing to shorten significantly or even skip a run or two. If you're having that problem you're likely running too hard.
Some years ago when I was running marathons in the 3:00-3:06 range, I'd have been in my late 40s then, Barry made an off hand reference to having me do "quality" two hour runs in which I might cover 21 km. That's about two minutes per mile slower than I could run for a marathon and he called it a "quality" two hours. So yes that was probably somewhere along the lines of 1/4th effort.
Arthur never liked the idea of long slow distance and wanted you to run as fast as you could without getting out of breath. But he also knew that people tended to overestimate how fast they could run without getting breathless so he also said things like, "You can never run too slowly. But you can run too fast." But you wouldn't avoid running fast if you can do so aerobically and you wouldn't force yourself to run faster than you can manage while staying aerobic.