Turns out it was too much.
I actually managed my time badly (Who, me???) and got a late start, so I had to cut the run short to make sure I wasn't riding home in the dark. It's a good thing I did. I only ran 12 miles, and I was dragging around mile 8, which is about the time my toe caught a rock and I went flying and landed face down on the trail. I was bruised and scraped up on my knees, arms and face, and covered in dirt from knees to nose! I dusted myself off and finished the run and ride home, but today I am just exhausted and sore everywhere! I assume the general all-over fatigue is from the workout itself, and the soreness is from the fall.
But I actually did have an incredibly enjoyable ride, even on the way home, when I was exhausted and sore. Perhaps it was because my only goal was to get home, just like my only goal on the way there was to reach the trail. And that's when I realized that, although I do love cycling, I don't have the heart of a cyclist. What I have is the heart of a Bike Commuter. I love to ride when the goal is just fun and fitness, and especially, to get somewhere. To get to work, get to the trail, get to the gym, or just get to lunch. In regular shoes. On platform petals. That is my kind of riding.
Unfortunately, I sense that is not the kind of riding that leads to a successful Ironman. So I will actually have to do goal-oriented, speed-specific, challenging rides. A lot of them. But I will make a concentrated effort not to lose my bike commuter roots, and every once in a while, put on my running shoes, slide casual capris over my padded shorts, and ride the 15 miles to Sunday brunch.