I took a good look at your running log. It looks like the longest unbroken MAF base period was about 3 months  from Dec to March 09. I only see a few runs exceeding fifty minutes during that time, and just one exceeding one hour (1:08-). Almost all the runs are about the same amount of time and in the recovery range. You pretty much maintain the same aerobic speed, as reported. You say you improved a little bit after a racing season.

My interpretation would be that you had just enough training load to maintain your aerobic speed during the base period. At those volume levels, you would need more time to see progress. MAF basework isn't some magic land that automatically generates aerobic speed. Other training principles still apply. Optimum training loads must be worked out. An amount of time on your feet that tires out enough of your fibers but that doesn't sacrifice your health. That doesn't mean you have to go run 3 hour long runs on a weekly basis, or ever. If you structure your week right, by the end of it you should have exhausted most of your fibers. A long run is essential. Maffetone, Daniels, and John Parker all cite studies that show that two hours is the most optimum time on feet for a long run. Two and half hours tops. There is a little gain at a half hour, more at an hour, more at 90 minutes, etc. But 2-2.5 hours tops. Anything longer than that is all mental work, and you are just pounding the body with no extra aerobic gains. If your long run is 50 minutes, you might experiment with adding 5% to one run a week, and 5% to total time. Making a goal of something a little longer. Whatever you do, take your time, and don't raise your time on feet too quickly. You look like you take enough rest--that always applies. Hard--easy/rest.

You saw a little gain in aerobic speed during a race season. That's not uncommon. Though there will usually be a plateau at some point, and if your anaerobic load is high enough, some regression will eventually creep up.

It really comes down to experimentation in loads, both aerobic and anaerobic, and staying in touch with your MAF tests. Whatever it is you commit to, go for it, stick to it, and truly see if it works (gaining aerobic speed, staying healthy, better race times).

--Jimmy