Going into the Tacoma marathon, I finished my dress rehearsal long run, which, as prescribed by Coach Greg McMillan, was a fast-finish 16-miler with the last 10 miles at or above my goal marathon pace of 7:15. My goals for Tacoma were: BQ (naturally), which required around a 7:15 place, negative split, not die. So based on my performance in the dress rehearsal, a BQ was likely, but what about the negative split?
Last year at Tacoma I was 4 minutes slower in the second half than the first; my theory was that it was based either on poor fueling or excessive pace (probably both). The former is easily corrected, but what about the latter?
The problem is that I have no clear idea as to whether my goal pace and negative split are corresponding goals. If I run at my goal pace, is that also excessive?
Here's what I mean: all of the articles say, in essence, to start slower than your goal pace for 3-5 miles, maintain your goal pace through mile 20 or so, and speed up to faster than goal pace for the last 10k. Simple, right? Here's the problem: what if your fitness level doesn't equate to your goal pace?
Let's consider two extremes. If I want to qualify for New York, I need to run a 2:55 (roughly 6:45/mile). Now, in my training, I'm usually running a 7:40 during my long runs. So if I start out at, say, 6:55, I'm technically starting 10 seconds slower than my goal pace, but of course I'm wrecking myself just to hit that time. So the notion that I'll be able to speed up to my goal pace just because I started slower is unrealistic. The second example is the opposite. If I walk the first half of the marathon, I can easily negative split because I can run at some faster pace for the second half than I walked for the first.
What these examples illustrate is to pace for a negative split, you have to base your strategy on some starting point that matches your actual fitness level.
Anyway, I ran across this article today and found it very illuminating. The article presents the case for determining when you'll hit the wall in a marathon. Essentially, it says that if you run at 85% of your V02 max, you will hit the wall at around mile 21 or so, depending on your glycogen stores. So in order to run a negative split, you need to ensure that your pace won't cause you to hit the wall during the second half or, at the least, avoid crashing into it at full speed. In other words, run at a reasonable pace during the first half such that you still have a solid supply of glycogen and make sure you're replenishing consistently so that when it's time to ramp up the pace, you'll still have energy in the tank to finish. But as I said, it all comes down to your fitness level.
So ultimately, the challenge is to plan for a pace that pushes you without decreasing your fat:sugar burning ratio so significantly that your system starts to shut down during the second half of the race. Unfortunately, your physical upper limit may not match up with your desired goal pace. It's at that point when you need to reset your expectation and remind yourself that just finishing is an accomplishment in and of itself, and a fast pace is just gravy. A BQ goal pace is an externally provided target that is independent of your ability. Your goal pace needs to be personal to you and how you are performing leading up to the race. Only when you base your goal pace off your actual fitness level does a negative split become a possibility.