An update: I’m more convinced the issue was overuse as a result of back-to-back hard days. I’ve been back running for a couple of weeks. Last week was a recovery week, and I got in about 40 miles and 4,000 feet of elevation without issue. I was pretty spent on my Sunday recovery run, but that’s to be expected. My back is feeling well, and I’m planning on ensuring I don’t somehow end up with back-to-back workouts on my calendar again.
With 8 weeks to race day, I’m looking forward to getting 4 or 5 more weeks to progress and then taking what I learned from this cycle and applying it to the next, which will definitely include more core and hamstring strength training in the base building period.