Just before I started my first round of marathon training back in August 2009, I had a metabolic/HR assessment done. A company called New Leaf has some software that they market to various third parties that will give you a very accurate picture of what is happening during exercise, and my gym happens to have the software.
The test is pretty simple. What you do is you put on this thing that looks sort of like this:
except minus the helmet and with a plastic tube sticking out. This plastic tube is then attached to a machine that monitors the composition of your exhaled gases. Plus a heart rate monitor. You then get on a treadmill and run at a reasonable pace while the tech raises the incline. The software and tubes and machines somehow magically figure out how much fat versus how many carbs you're burning while you're running. The tech raises the incline until you hit what is called the anaerobic threshold, where you start accumulating lactate in the blood (and at which point you're burning 100% carbs, as science would have it). And then you stop and walk for a while to recover.
At the end, you get this nifty set of numbers that tell you what your five HR zones are. This is important for training purposes. Zone 1 is pretty much a nothing zone - I don't spend a lot of time here. Zone 2 is a zone for teaching your body to fat burn - this is where I like to spend most of my time when I do long Saturday runs. Zone 3 is a zone for endurance - most of my "junk miles" are spent here, and I also race at the very upper threshold of this zone. Zone 4 can't be sustained for a long time - the anaerobic threshold corresponds to the boundary between Zone 3 and Zone 4 - and is where I train to get faster in interval training. Zone 5 is flat out booking it and I might - might - get here in some of my harder interval work.
You also get a pretty accurate picture of how much fat you are burning at what HR level. This is very helpful for controlling diet (because you get a clear picture of what your calorie needs are) and for trying not to bonk in marathons (if you can accurately estimate how much glycogen you can store, which is admittedly guesswork).
The thing is, all of these numbers change over time based on training. So the test I took in August of 2009 was very different than the test I took in January 2010. And that one will be far different than the one I will take tomorrow. Which is why I bring this up in the first place. I'll compare numbers tomorrow night, most likely, after I get my test results back.