Leslie,
That is a great question. In 24 hour mtb races we often dig into zones above zone 2 just to get around the course. You can be a little more controlled in your road 24 event.
Heart rate is a physiological response to exercise (or more specifically power to the pedals on a bike) and not a precise pacing tool. Heart rate response to increased power to the pedals varies during a 24 hour race. It is not consistent. It is somewhat predictable though.
Heart rate becomes depressed for a given power with fatigue. In an endurance event this starts to occur at the 3 hour mark and will decline to the 6 hour mark then level out.
What does this have to do with your question? Well, during the first 6 hours of your event you can increase power to maintain speed into a headwind – and your heart rate will respond nicely rising into zone 3.