For someone who has been running since his pre-teen years to be within 4 minutes of his half marathon PR at age 55 seems astounding but not if one considers what you've offered about your training history here and what I've picked up of it elsewhere.
There's a compliment in there somewhere. Actually, I'm not within 4 minutes of my half PB, just the best time I ran back in the early-mid 90's. My half PB is soft as I ran 1:27:10 en route to my marathon PB back in '83. I suspect if I'd ever run a half back then it would be sub-1:24. I was floating at that point, and due to my minimal miles was much better prepared for it than the marathon.