To improve your fitness and your swimming efficiency, you need to develop pace versatility in your training. Actually, there is nothing wrong with LSD itself; it’s a great way to relax after a hard day’s work - you get to put all the tensions out of your body and into the water - or just to enjoy yourself in a pleasant aquatic environment.
But, if all you do with your swimming is plodding up and down the pool or leisurely enjoying the delights of the natural environment in the open water, you will never improve your fitness.
You need to mix up LSD with more cost-effective or value-added training, such as varied cardio intervals. That is why we concentrate on such training in our Sunday swimfit sessions; you do your LSD swimming mid-week.