Update:
Week 1: holed up in hotels and looking at weather out the window, but started a mild routine of laying flat on a foam noodle while doing sit-ups, crunches, upper body exercises. I had some residual pain from last weekend, so went easy on the routine. Went for a ride today, making sure to do the stretch and exercise routine first. Made sure I was out of the saddle on climbs, paid close attention to riding position, and noticed something else. The bike is new, as is the Brooks saddle. The saddle is a bit slippery, and there is a tendency to slide forward on the saddle, which makes a difference in position and where stress goes. Being aware of this, it wasn't hard to push toward the rear of the saddle, which was more comfortable, gave more power, and took stress off the lower back.

So far, so good.