You can build swimming endurance by performing long swims, one or more times per week, depending on your swimming goals.

Long swims are done at your endurance pace (EP), which is 4-6 seconds longer per 100 than your threshold pace (TP).

• If your 100-yard threshold pace time is 1:30 or greater, your endurance pace time per 100 yards is 6 seconds longer than that.
• If your 100-yard threshold pace time is less than 1:30, your endurance pace time per 100 yards is 4 seconds longer than that.

You should feel that you can maintain your endurance pace indefinitely. If you are new to long-distances, swimming at this pace may feel too easy, but it improves virtually all aspects of cardio-vascular performance by increasing the capillaries around your slow-twitch muscle fibers, which facilitates the delivery of oxygen and nutrients, and speeds the processing of lactate.

Marathon swimmers and triathletes should spend the bulk of their training time at endurance pace.

Short-course triathletes need to do some work at endurance pace, but not much. And swimming sprint racers should spend very little time at endurance pace, perhaps using it for short recovery swims or technique-focus sets only. If sprinters spend too much time training at endurance pace, fast-twitch muscle fibers may switch to “slow-twitch” mode, reducing sprinting speed.

You don’t need a warm-up or cool-down for an endurance workout. Just set your timer (if using one), and go.  But you may want to do some technique drills before setting off.

Build endurance training volume gradually, by about 10% per week (assuming you perform one endurance swim per week).

You may prefer to do your endurance swims in open water, but I strongly suggest that you learn your endurance pace first, at the pool.