I am just a Navy guy, but how many Marines have a 16:12 PR for the 5K? Heart rate training, intervals , etc all have their place in making you faster... but I find that the most often ignored aspect of runner's training is power. People think for long distance running you don't need it, but power is a component as well. You need to step back and look at your program's strengths and weaknesses. If you need power workouts, try running hill repeats and get in the weight room. Find a running/triathlon specific weight workout. Best of luck.