I think taking yesterday off was the best thing I could have done. Not only did I wake up feeling peppy again, but I have a little more clarity about my training. Specifically: All I need to worry about right now is losing some weight and biking, biking, biking. It's too soon to worry about a formal training schedule, and it's way too soon to freak out. (Or maybe way too late, but there's no getting out of this now.)
I did have an insight on the treadmill that might be helpful: I walk way too slowly. And since undoubtedly the Ironman is going to involve a good amount of walking, it would be really good to get comfortable walking faster. So today I pushed my pace a little bit, and next week I'll push it a little more. It wasn't my-mom-walking-through-the-mall fast, but it's something.