Here's the results for a month ago, and for yesterday:
You can see that my paces have improved slightly in the last month. They are as good as they have ever been since I started this kind of testing. As long as you have the same conditions you can see quite small improvements with this testing method without racing.
I'm not sure if these numbers will continue to improve, in any case I'm now very very close to having the <4hr pace at very low aerobic HR: To run a sub 9:09, I would probably need to run about a 9:04 on the garmin and this is only 15s/mile faster than the MAF pace.
Currently my speed up vs HR is about 13-14 bpm per min/mile, therefore for 15s I need about 13 * .25 bpm or ~3-4bpm...so my target HR would be 125 + 4 = ~130bpm (at the race start before drift occurs).
This is not a very fast HR for me since my max HR is 168. (tested on a treadmill by a Dr).
The main issues then are LT (need to push that up well above 130bpm so we don't fade) and basic long run muscle endurance/strength.
Really trained marathoners can run races much closer to their max HR: for example 145 bpm for my max HR would be a possible goal. This is a full 15 bpm higher than my 9:05 pace HR..and would would give me another min/mile. A BQ even?!
The fly in the ointment is getting the muscles to actually do this...IMHO, the heart is actually not the problem at all in the marathon. It's all about the legs/muscles/mitochondria etc.
Anyway, I'm encouraged at the progress in my running economy so far. I will do one of these tests every 4 weeks or so until CIM.
I personally like using the HR monitor but for people that live in very hilly areas it would not be nearly as useful.