Ethan and Mattie discuss "coasting". Why is this significant? What might this possibly represent or foreshadow?

"This is the night we were to have gone coasting, Matt," he said at length, with the rich sense, as he spoke, that they could go on any other night they chose, since they had all time before them.

Ethan and Mattie discuss sledding..... something that foreshadows the accident that will affect the rest of their lives.