I think once you reach the challenge, it registers as done permanently. So once it's "done" on easy, it'll stop registering your distance, thinking the challenge is already done. I'm really not sure how to make a workaround for that... I'm still pretty new at this. This is my first actual mod (Not counting the tweak just for alien vehicle distance).
EDIT: Note that this is just my speculation. I could be wildly wrong. Like I said, I don't have too much experience in this. The whole purpose of this is to try to teach myself, and maybe help out someone else along the way. If anyone knows what could be wrong, or how to fix it, and wants to help, shoot me a PM.