First, HR is an indicator of the number of beats per minute your heart is beating. See above. The end. Your body will let you know when you're done much more accurately than a HRM. Use the search engine here and you'll find multiple threads on this topic.

If you find yourself out of breath, you need to improve your aerobic capacity. If your legs load up you need to improve lactic tolerance. If the legs get tired you need to improve muscular endurance.

You'll find that if you stick with the sport your limiter will move around the three points above. Each can require a different type of training to improve the limiter. Use the search engine here and you'll find multiple threads on this topic.