Squad Swimming For MX Training
I found the quickest way to get "cardio" fit was to join a swimming squad. The coach on the side of the pool structures the session - and any swimming coach worth his/her salt will devote some of the squad swimming to hypoxic training. This is when you experience an oxygen deficit during swimming, and will quickly start working at your maximum heart rate.
It's not so much about getting to max heart rate that determines fitness, but the ability to recover (drop your heart rate) quickly.
After a warm up, try swimming 12 x 50 meters on the minute, with tumble turn (assuming a 25 meter pool), and breathing every 3rd stroke. Initially you should be finishing 50 meters in 45 seconds (or faster), which gives a 15 second rest before you go again.
When you're unfit, that 45 seconds will creep closer and closer to the minute, which is a sure sign that you're not (yet) able to bring your heart rate down swiftly.
It is not necessary to monitor your heart rate; rather watch your split times. Close to the end of the session, if you're swimming consistent laps under 45 seconds, (or whatever target time you chose) then it means you're well on the way to becoming really fit.
Swimming like this alone requires much personal discipline and focus. I find it much easier to surrender to the coach, and simply do what they say. Also, swimming with others around you brings out one's competitive nature. At first, this hidden agenda (it's a race!) can be very intimidating, but if you can swallow your pride until you get fit, you will reap massive benefits next time you go biking.