Swim in legs. But don't use legs
Split your swimming target into separate legs to boost your endurance and improve your fitness. Oh and don't kick too much, use your legs mainly to balance - it'll help you last longer.
Many swimmers struggle to increase their swimming distance. Adding more to your total swim yardage in the pool or in open water requires more than swimming to exhaustion in each training session. To improve your distance, concentrate on improving your endurance in and out of the pool as well as honing your technique to improve your efficiency in the water. Here are some steps you can implement to get that distance back up;
There are two basic approaches to increasing swim endurance in the pool. The first, and least effective, is to swim the target distance without stopping. If your goal is to swim 2,000 meters, you swim 2,000 meters in the pool using every stroke and kick you can to cover that distance. The downside to that approach is that your technique degrades quickly as you tire. You will ingrain poor habits and see little improvement in your time.
The second approach is to divide the target distance into smaller chunks, gradually increasing their size as your condition improves. For example, 2,000 meters can be divided into twenty 100-meter swims with progressively shorter recovery times between each set of 100 meters. The swim sets can then work up to ten 200-meter swims and four 500-meter swims. The progression would continue until you can swim 2,000 meters without stopping to rest.
Most long-distance swimmers use the freestyle stroke, and improving your technique will provide a significant return on your ability to swim longer distances without fatiguing. In every training session, spend at least 10 to 20 minutes working drills that will make your strokes more efficient in the water. Examples include one-arm drills, using hand paddles and pull buoys and focusing on maximizing arm extension throughout the stroke.
Swimming longer distances involves very little effort from the legs. Since the legs are comprised of large muscle groups, the more you use them in swimming, the faster you will fatigue. When working on kicking technique, focus on minimizing leg movement. The legs are used more for balancing the body horizontally in the water than propulsion. With this in mind, avoid using kick boards since they will teach you to kick harder than you need to. Instead, place a pull buoy between your knees and note how little effort you need from your kicking to move forward. Alternate 50 meters of swimming with a pull buoy and 50 meters without one to develop a feel for the minimal amount of leg movement you need to keep your body in the proper position.