Arthur used to say that slow running would get you fit but it would take longer than running at your best aerobic speeds. There's a video of him running with Heather Carmichael, I believe, in which he's telling her to slow down and relax because "distance matters more than speed." He told Joe Henderson that "my runners ran as fast as they could without getting out of breath." Of course the flip side to that coin was that they ran as slowly as necessary to avoid getting out of breath. In "Running to the Top" he wrote "You can never run too slowly. But you can run too fast." The only advice he ever gave me about pace on distance runs was "Enjoy yourself."
He said lots of things about pace on distance runs. There's maybe so much variety in those comments that they almost can seem contradictory. So here's what I learned from talking with and writing to him and doing the same with Barry Magee. You should enjoy your distance runs. They should be comfortable and relaxed. They should leave you "pleasantly tired," not anything close to exhausted. But as time goes on and you get fitter, those things can and likely will happen at faster paces and you should allow that, but absolutely not force it, to happen. Make mileage your top priority and run as slowly as you need to so that you can do that mileage week in and week out. But don't run slower than that. If you need to run 8:00 miles to get your 90 minutes in each day, run 8:00 miles. Don't worry about forcing yourself to something faster. But if you can run 6:00 miles to get your daily 90 minutes in and really can manage that week in and week out there is no need to run at 8:00, or 7:00.
But he would have preferred that you not fixate on pace at all and instead use feel as your guide. If you are going to err on the side of either going too fast or too slowly he would recommend that latter, because again, "You can never run too slowly. But you can run too fast." And also again, he believed slow running could get you fit but in a longer time than faster runs would. If you're an out of school runner who doesn't need to be ready for a season that starts on a particular date or a national class runner needing a qualifying time for something like an Olympic Trials, where's the rush?