With race season fast approaching and the soreness in my hip still nagging, I decided to seek more professional help.

This time, I saw Chris Roy, who runs Active Body Studio in Nashua. He's a pleasant, no-nonsense bloke, with what looked to me like a very well equipped gym (mind you, this was the first and only time I'd ever visited a personal trainer).
He told me more or less the same thing I'd heard from his neighbor, sports massage therapist Kathy Farwell... my chickens were home, comfortably at roost.
I used to tell myself, years back, that I didn't have a beer belly, I just had slack abs. In truth, I had both. I'm fitter now, but years of neglecting my core left me vulnerable to back pain and all sorts of other issues, including overtaxed hip flexors.
Beyond getting some advice for my own situation, I learned a couple things from Chris that might be of use to anyone. First, when stretching a muscle, it helps to also "activate" or load the muscle by making it work. If you are stretching your hamstrings by pulling your leg up toward your torso, for instance, stretch a bit and then use your leg muscles to resist, pressing in the opposite direction. Then relax, stretch again, and you get an even deeper stretch. It seems pretty obvious in hindsight, but I never knew.
The other thing I learned is that those six-inch diameter foam rollers are worth it. I'd been using foam swimming noodles, and sometimes a shop vac attachment tube, to roll around on my IT band. The noodles are too soft. The vacuum tube is too hard, and both are too small. The purpose-made rollers are just right, and work really well for rolling around on the piriformis, too. While waiting for the roller I ordered, I've been using a softball. It works pretty well, too.