Pool membership or swim instructor?

Ultimately you are going to have to put in the yards/meters, however, you can swim hundreds of thousands of yards/meters and if your technique is flawed you may have largely wasted your time.  

Technique and swim fitness are two sides of the same coin.  You need both, but here's the thing - you can't have good technique without good swim fitness and you can't have good swim fitness without good technique.  That's why swimming a lot with poor technique is largely a waste of time - without good technique, you aren't developing good swim fitness specific to the muscle groups you need for proper technique.  It's a bit of a catch-22.  You need to develop proper technique simultaneously with building proper swim fitness.

The best thing you can do to improve is get with a QUALIFIED coach/instructor.  There is no replacement for eyes-on-deck.  As you begin to improve your technique, you'll begin to develop proper swim fitness.  Once your technique is good, then start building the volume.