If you don't really care whether or not you thruhike, chances are good you will not finish a thruhike. It takes a pretty strong desire to finish to keep you going when things gets hard. That's true of all the long trails. I've seen a lot of people quit because they simply didn't want it enough.
On our first CDT hike, we started the trail with a younger much more fit and experienced hiker. One day he said, "If it were just a matter of being fit, I'd be the one sure to do the whole trail. But I don't have the fire in the belly that you two have. So reality is, you're a lot more likely to do a thruhike." He was right. He got off the trail at Yellowstone.