I've got very fast before on almost exclusively easy running training for an ultra, but we are talking 15 hours a week at least. You can obviously cover intensity, with huge volume. But at thie point in my life I have no time for it.
I restarted training via NSM on 5 days, the 3+2 almost beginner at around 4.5 hours a week after nearly a year out. Built up over 19 months training like this to 8 hours a week, still on singles.
Ran a lifetime PB at 10k today and I expect to run a lifetime 5k next weekend. Over the last 19 months, I've probably ran more consistently in terms of load generated in that time than even equivalent periods when training for ultras over the last decade. It's shocking how easily consistent weeks of the same thing over and over stacks the fitness over time.
I'm glad I found NSM as I just don't have the time to run that much anymore, wife, kids etc and i can be incredibly competitive for my standards still on 50- 60% of the time i previously committed.
There's probably different ways to make small adaptations per individual, but in the end it's all about spreading the risk and management of load. You can't have everything but NSM prioritizing the foundations of training without all the jazz is the key to it.
What sirpoc has done and probably changed things for a lot of hobby runners for the better, is challenged what is important versus what is not. Or just deciding in a situation onlf limited hours what is important versus what you can do without.