You have taken the first step to improve your performance. The next step is to know your training zones and then develop a training program. I do want to highlight the fact that your maximum heart rate doesn't really play a role in developing your training program. What you need to find out is what your threshold is.
You have 3 options:
1. Self test - Warm up, do a 5 minute interval as hard as you can, but maintaining your pace for the entire 5 minutes. Recover from that interval and then do a 20 minute interval as hard as you can but still maintaining your pace. Take the average heart rate over the 20 minutes and multiply that by 95%. That's your threshold.
2. VO2 Max test - You can also go to a qualified lab to get your VO2 max test done. You will also get your threshold when doing this test. This can be a little expensive ($150-$500) but they are very accurate.
3. Hire a coach - they will help you determine your threshold.
Once you determine your threshold, you can calculate your training zones. Most coaches have their own "zones" so the percentages might differ a bit. Here are some typical percentages you can use:
1. Active recovery - <68% of your threshold HR
2. Endurance - 69-83%
3. Tempo - 84-94%
4. Lactate Threshold - 91-105%
5. VO2 Max - >106%
6. Anaerobic capacity - Max HR (as hard as you can go)
Once you know your zones you can then design a training program to help you improve your endurance. Remember to increase your volume slowly, no more than 10% per week. Good luck and hope to see at the races.
