Slow down and try to get your 5-minute pace. That website will base your future workouts on your pace for various distances, so it's pretty important to feed it accurate information.
Generally you are looking for a lower stroke count and faster speed. In fact, an excellent drill is called "swim golf" in which you add your stroke count to the number of seconds it takes you to cross the pool, then work on reducing that "score" via fewer strokes, faster speed, or both.
edit to clarify a little - Think of your pace for "about 5 minutes" as being roughly the same as your run pace for "about a mile." It wouldn't make sense in that case to sprint for 1/4 mile and then die. Same thing here.
- Le Bannon