Focus on technique. It will do more for you than muscle alone.
How well you can swim brings more success than how long you can swim. Practice technique more than endurance.
Especially work on starts and turns. The fastest you will ever be in a race is when you dive in. The second fastest is your push-off/glide on turns. You are slowest when swimming. Every turn in practice, whether in warm-up or hard sets, is a chance to practice and perfect your technique. Take advantage of that time.
And don't be shy about asking for advice from coaches and fellow swimmers about how you can improve your technique.