If you exit a round before you are finished, take a rain check, or leave the course before completing an entire round, you can easily resume your round where you left off. On the round screen, the round will have a “Tap here to resume this round” prompt above the course name of the given round. By tapping this prompt, you will be brought back to the hole and shot you were on.
If you only played 13 holes in a round, you will not generate personal bests, unless you achieve a new longest drive. Your scoring statistics will be added to your profile, and a handicap for your partial round will be generated as accurately as possible.