Chances are you wear a heart-rate monitor on the bike and run but rarely in the pool. So how do you know you're training at the right intensity? One alternative to using a monitor is to measure the intensity of your training using your rate of perceived exertion (RPE).
Using a scale of one to 10 (or one to 20), this method rates effort number one as complete rest and number 10 (or 20) as your absolute maximum intensity. Although this does take experience (and honesty!) it is an easy and efficient way to monitor your training effort.
On a scale of one to 10, a three would be a very easy session, perhaps a very easy distance swim, run or ride with concentration on technique; a seven would be a medium-intensity session; while a nine would be a demanding interval session with the emphasis on quality and hard work.
Use this test to give yourself an idea of what your RPE might be in swimming.
1) Swim 50 metres fast (a level 9, almost flat-out but not quite), then rest for two minutes.
2) Using the time from your first 50 metres, add 10 seconds to work out your starting pace (this is level 5).
3) Each two seconds of going faster adds one level of effort to your RPE.
4) You can use this starting pace as an easy to medium intensity to begin your longer distance repetitions.
For example, if it takes you 50 seconds to swim 50m, add 10 seconds to this and your starting pace will be 60 seconds to swim 50m at RPE 5.
Swim four 300m efforts completing each in six minutes (50m x six x 60 seconds) at an RPE 5. Rest for one minute after each 300m set.
As you become more tired, you will have to swim harder to maintain your target time of six minutes for each 300m set, and you may finish with a final effort at an RPE of 7 or even 8. This type of training session is an excellent way to begin to build up swimming endurance.