It took me a few extra days (plus I started a couple days early), but I just finished the 100 miles!
Dr. T said yesterday that it seemed like this challenge got me to realize how walking regularly can be helpful to me. He said it was a bit funny that after 5 years with him, it took this challenge, as he often mentions exercise (the past few years, he'll just mention it in passing, not push me on it). But that he's glad I eventually realized it.
It's been helping with sleep (both quantity and quality), and my resting heart rate (according to Fitbit) has gone down considerably, like by 10 beats per minute in 5 weeks. Now I just need to continue it. But seeing actual improvements like that, with numbers to back it up, is a good incentive to keep with it.