Lets go back to optimizing performance. I have several a 3k's, I have ran several 5k's, and have ran one 10k (thus far). I never ran out of breath in any of these three events. Meaning that my limiting factor was not my cardiovascular capacity. Why? Well, because I had already developed an aerobic base. If you develop that aerobic capacity and remain in an exercise program that aerobic base will not diminish. As a matter of fact, my breathing was the same from that 3k to that 10k. I did some reading and found that runners that run a 10k and a marathon show no change in VO2 max levels. This means that when making that transition form a 1ok to a marathon the VO2 levels are unaffected. Why? the individual is already in that oxidative pathway and there is no need to change. This means that if you have already reach VO2 max levels at 10k your body will get nothing out of running those extra miles. Running more that 10k will not improve you VO2 max. THIS IS A FACT.
What was the limiting factor? My legs were on fire. That burning sensation meant I lacked muscular endurance. In other words I was weak. So what's the remedy? A perfect combination of CrossFit and to make yourself work faster than normal. Fast running, sprinting, intervals, etc. Your training regimen should focus on speed training and maintaining the ability to recover. A great example is running 4 x 400m and focusing on maintaining a desired pace with minimum rest in between. Maintaining the ability to recover is important if you cannot recover (meaning you cant bring your HR less than 120) in less than two minutes the work out is done.
If you are interested in running a marathon or a triathlon. Please don't hesitate to ask. You cannot expect to run 40 miles a week, spend time with your family and friends, and work exercising 30 hours a week. Time to be efficient. I would be more than happy to help and develop a program that will improve your times and improve all the other components of fitness (strength, power, agility, etc.)