Anerobic Threshold and Workout Optimization

Hi, Relatively new to swimming - only started last May, but have become obsessed with the sport. So much so that I have done a fair amount of reading on how to best workout and properly train the correct energy systems for the appropraite purposes (sprinting, middle distance, and long distance events). I have not yet competed, but want to, perhaps next year. I am a 47 year old male, 5'11"+, about 195 Lbs. (need to lose about 10-15 more, but have already dropped about 35 lbs since I started swimming). From my reading, apparently one key to proper training seems to be determining your Anaerobic Threshold speed. This will help you determine your EN1, EN2, EN3 training paces. But I am confused by all the different ways that it can be calculated, and what appears to me to be able to produce wildly different results, even using the same method. Any comments on this point? And what method (except for actual blood testing) do you recommend?