Intervals -- why am I doing this again?

Today I got bitch slapped by reality, or actually the weather, which is one variant of reality. I was planning to do a long run of at 16-18 miles, mixed in with 10 1 mile intervals. It was raining slightly, or 'misting' as we like to say around here (in Seattle we have many different terms for rain), nothing too hard, but it was 35 degrees, so I put on a shell instead of a tech tee, strapped my headlamp on (thanks to Daylight Savings, it is now pitch black at 6:00 AM), and headed out, full of ambition.

I had downloaded the days workout into my Garmin. 10x1 mile at 8:40. I felt a little slow going out, but chalked it up to the disoriented feeling I was getting running in the rain, in the dark. It is fairly surreal -- my headlamp lights up a small cone of streaking raindrops, and very little else, so I start to zone out (and slow down).

When I started the intervals, I knew something was wrong. My get up and go had got up and went. And it was raining harder. No more mist, more like psuedo hail. Then snow, big fat flakes that chilled my face to numb as they fell and stuck. Still, I soldiered on, trying to maintain a pace that seemed pretty achievable from the glow of my laptop that morning, but was proving to be a painful challenge on the hilly Mercer Island loop.

A younger and less intelligent me would have ground myself down into a little nub trying to do the intervals, but I'm older, and less inclined to hurt for no reason, so I quit. I turned around, changed my plans from 18 miles to 11, and packed it in. I ran a couple of intervals on the way back, but by then my legs were so frozen that it felt like I was running through (very cold) molasses.

Later, in the shower, I analyzed the workout. Heres the thing. I know I'm fairly motivated. I know that I'm working hard. My heart rate told me so. And that ended up being the key. Why am I trying to maintain some arbitrary pace up and down a hilly course? I should be running these intervals by heart rate -- by setting a target to stay above -- or below, depending on the goal of the interval -- I'll run at a specific effort, and the pace will be what it is. This feels a lot more real than trying to do intervals at X:30/mile 'because so and so said to', or because X:30 means I'll run a marathon in Y:45. Those are completely arbitrary, completely false goals, and I will crash and burn trying to make them work. On the other hand, I firmly believe that if I can run to a specific heart rate, I will see improvements in speed, because I will see improvements in efficiency.