I've just completed the Newport Half Marathon and had an pretty steady heart rate of 88% of my max.
My details are here.
Is this good? Generally speaking, what should the heart rate be for a half marathon?
Your max heart rate will happen typically at the end of the marathon, when you're making your final push for the finish line. Comparing your heart rate throughout the majority of the marathon to this maximum isn't very meaningful.
Maybe you just worked really hard right at the end and you'd end up with your steady heart rate being a lower percentage of your max. Or maybe you don't push that hard at the end, so your steady heart rate is not that much lower than your max at the end.
I could give more examples, but hopefully you see that there is no meaningful conclusion that you can draw between the ratio of steady-heart-rate to max-heart-rate.