Marathon and Aerobic Capacity

Given that the primary focus for marathon performance centers around one's ability to maintain fast paced aerobic running over the duration of a marathon I have some questions. I would like to preface my question(s) by stating that I feel in my personal running that the one element that dictates my performance centers around what appears to me to be my oxygen capacity. In any race I always feel that the greatest limiting factor for me is my ability to utilize oxygen. During a race my lungs feel like the limiting factor. I don't ever feel that I don't have the fuel or the muscles for a faster pace, but that my lungs (breathing) simply won't allow for a faster pace. The exception to this was my one and only marathon where I would say that my legs got heavy around mile 20. This race occurred after only 2 four-month training cycles. At the time of my marathon I had been running for around 10 months with 1 three-week break between cycles. I have continued training since then and usually average between 45 and 65 miles per week. Given this, I am going to assume that my muscular development has improved and will probably be stronger for my next marathon in April. Still though, I continue to feel that my lung capacity (again breathing) is my limiting factor. My basic training plan consists of about a 5 month training cycle where I usually focus on Steady-State running for the first half of the cycle and begin to add tempo runs in with increasing frequency and distance covered over a single tempo run through the 2nd half of my training cycle. I utilize hill repeats as well, but not usually more than once every three weeks throughout a single training cycle. I don't really focus on any interval repeats or V02 max workouts, preferring instead to spend the time that could be used on such workouts (V02Max) on tempo runs. Finally, I should add that most of my runs, regarless of distance are usually done in 6:50 - 7:15 range. A slower recovery run is usually run in the 7:30-7:40 range (but this pace range is usually run only once per week. Over the last year I've dropped over 5 minutes off my 1/2 marathon time and am shooting for an improvement from 3:11 to 2:59 during the year between my first marathon and second. I have read that general aerobic fitness will improve continually with consistent running for approximately 10 years. I've now been running for about 18 months with regularity (usually 5-6 days a week, but some weeks I'm only able to get 4 days in -- but generally this is an exception). Am I doing the right thing to improve my ability to utilize oxygen for marathon and 1/2 marathon performance, or should I be adding in regular V02 max sessions? How can I improve my ability to overcome what I feel is my greatest performance limiter?