Watch the result be something ridiculous, like the truck just bounces off her enormous boobs.
Or maybe she'll be like, "wow, my fat cushioned the blow--I don't want to lose weight anymore!" and she stays thick the rest of the run. But I suppose that's too much to hope for.
It's just gonna be something lame like, "wow, with my incredible superhuman speed from exercising a moderate amount for a few weeks, I was able to save her from certain death!"