Swim, swim some more, and when you think you've had enough, swim some more.
I, too, am an adult onset swimmer. When I started (April 2012) 25 meters was a challenge. Now I can go 2,000 meters plus without stopping. Having a coach is great for getting the technique right. Other than that it's about the volume.
Hi, have you found the Australian website swimsmooth.com
It's excellent for all levels. Like having a personal coach, they even send you weekly emails once you are registered. Download the animated swimmer to make him interactive, he will give you hours of pleasure.....