Enter a swim meet.
Competition is the key to staying motivated during training. Search for a local Masters meet online (Usms.org) and register as an individual or as part of a team. Pick the longer events and work on endurance and pacing and enter the sprints to improve speed and strength in the water. If a swim meet holds no appeal for you, schedule a monthly 1K time trial into regular training. Record the overall time to track improvement and use 100 splits to judge pacing ability.