If I know I'm going to be grinding into a headwind for a lengthy period of time, let's say an hour, I find the pace at which I know I can sustain it for that long. Any faster and I'll be worn out and wear down too soon, if slower I'll just be prolonging it longer than necessary. I use the same technic for long (century + ) rides - set the pace at the beginning that I know I'll still be able to hold at the end (granted, I take these long rides as "tours", not "races", so that may not be the fastest way to ride them, but it's how I enjoy doing it.).