Nice work. Perhaps training that way is a sweet spot...up to the Half distance. Honestly, I can't imagine that approach for a Marathon. Everyone I know who has tried the 'First' method for marathons, has fell short of their expectations.
Also - for me - I noticed that the more I ran, the less injury prone I became. Obviously there's a tipping point. But once I upped my mileage significantly and did it mostly in Zone 2, I rarely got injured after that. But I'm glad this works for you. If nothing else, I'd like that plan as it would mix things up. Biking would be a nice alternative when I get burnt out from running.