If you tap the Home button when you are tracing your race track’s path or dragging items onto the track, the track will not be saved. The race track you made will only automatically be saved once you start the race (you do not have to finish the race for it to auto-save).

To see your saved track, tap the Home button, and then tap Build.

You will arrive at the track menu, where you should see that one of the 12 save slots has filled in with the track you created.