Because I am not a very strong biker, and I really hadn't trained as much for the biking leg as I probably should have. The biking leg was 30k long. I did it, but it really took everything out of me. There was a hill right near the end of the course, too, which was absolutely brutal. I'm still not sure how I mustered the energy to get up it the second time (it was a two-lap kind of thing). When I got back off my bike and set out on the run, my legs felt like gelatin. That's such a cliche, but they were really wobbly, and it was all I could do to keep them moving. I hadn't kept enough "oomph" back from the bike leg to get me through the run. I did manage to run the entire distance (5k), but it was a very slow kind of run -- barely even a jog. I spilled Gatorade all over myself, but I suppose that's to be expected. I kept telling myself "don't walk... if you walk you'll never start running again... you can do it..." and calling on every ounce of stubbornness in my body. And I did manage to run the whole way. I even managed to run the last 200m or so at a proper speed -- and it felt like a sprint.
Oh, I've just found the standings. It appears I was not, in fact, absolutely last. I came in 273 of 277, including the three who didn't finish it. One person finished behind me. It took me 2 hours and 36 minutes.
But I did it!