Posted by Dave on September 30, 2013 | 2 Comments
This past weekend I ran a 15K race and was hoping to achieve a PR — finishing the race in under 60 minutes. It’s a good goal because it’s not only a nice, round number, but it’s also right at the threshold of my abilities. I’ve finished 5Ks in under 20 minutes and 10Ks in under 40 minutes, but I’ve never been able to sustain that pace for any longer race.
To achieve a nice, round, 20-minute 5K or a 60-minute 15K requires the same, not-so-round pace: 6:26 per mile. If a course is perfectly flat and perfectly well-marked, all you need to do is run 6:26 every mile and you can reach your goal.
But of course, most races have hills, and courses are often poorly marked, so runners rely on their GPS watches to monitor pace. But what pace should you run on the hills? Typically in the past I’ve resorted to guesswork. If mile 1 has a big hill, I plan on giving myself a little extra time, and then making that up later in the event. But this event seemed evenly hilly throughout, and most of the hills were short, often a quarter-mile long or less:
I decided on a different strategy: Instead of planning for each mile, I’d just set an uphill pace and a downhill pace. Then all I’d have to do is take a split at the top and bottom of each hill and I could almost run the race on autopilot.
To account for GPS error, I set a goal pace for the entire race of 6:20 per mile. Then I just added 15 seconds for each uphill section and subtracted 15 seconds for each downhill section. Since the race starts and ends at the same elevation, I should have just as much uphill as I have downhill, right?
During the running of the race, I wasn’t quite able to maintain the paces I planned: I was running the uphills a little fast and the downhills a little slow. But I figured that should probably even out and I’d still be okay. Except for one niggling detail: As I ran, I could also track my average pace for the entire race, and that figure kept increasing for the entire event. I was shooting for 6:20 per mile, and it crept up little by little — 6:22, 6:23, 6:24. I was quickly running out of wiggle room for GPS error. As it turned out, I’d need that wiggle room. In the end, my GPS measured the course at 9.46 miles, instead of the 9.3 I was expecting. At that distance, I’d need every bit of a 6:20 pace per mile. My finishing time was 1:00:37; I missed my goal by barely 1 percent. Even though my GPS put my average pace for the race at 6:25, I still didn’t finish in under an hour because of the small GPS error.
But after I got home, I downloaded my GPS record and noticed something interesting: If I took an average of the paces I ran on the uphill and the paces I ran on the downhill, it seemed like I should have been much closer to that 6:20 pace. The average of the paces for each uphill section was 6:31 per mile, and the downhill sections averaged 6:11 per mile, for a net average of 6:21. That might just have been enough to get me my sub-60 15k — especially if I saw I was close at the finish and made a final, mad sprint. When I weighted the averages to account for the fact that there were 5.44 miles of down and only 4.02 miles of up, my theoretical pace improved even more, to 6:20 per mile.
So why didn’t I achieve that pace in reality? It took me a while to figure it out. Imagine a race that runs over a hill and back: There are two ups, and two downs. If the hills are equal lengths, then my strategy works perfectly, even if I don’t run the exact same pace on each hill:
Here, my average pace on the downhills is 6:30 and my average pace on the uphills is 7:30. You can average those together and get 7:00 per mile for the whole race.
But now consider a course where the hills are unequal in length, like this:
Now the uphills are all each the same length and the downhills are different lengths. Suppose the uphills are 1 mile each — then I averaged 7:30 on all the uphills. But if the first downhill is 1 mile and the second downhill is 2 miles, then I didn’t average 6:30, I averaged 6:40 per mile on the downs, which means my overall average is worse than 7:00 per mile.
That’s what happened during my 15k. The slowest downhill sections were also the longest downhill sections (which makes some sense, since those hills weren’t as steep). Instead of averaging 6:11 on those hills, when I take the length of the hills into account, I actually averaged 6:21! Put that together with my 6:31 pace for my uphills, account for the fact that I ran a longer distance downhill than uphill, and you arrive at my 6:25 average pace, which wasn’t fast enough to overcome my GPS error.
I probably would have been better off just trying to run 6:20 per mile throughout, instead of relying so heavily on the up/down strategy. Of course, an alternative explanation is just that I’m not in good enough shape to run a 60-minute 15k on a hilly course! But either way, I think a straight-up mile-by-mile plan would have been easier to adhere to during the race.