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.
HR is a very accurate measure of effort, and as delayed as any other natural parameter, call it breathing, perceived effort, fatigue (or even sweat to give another example).
Your heart doesn't know if it's cold or hot, uphill or downhill. Any factors that increase/decrease your heart rate will likely impact your performance
The only thing that might be important is to normalize versus minimum heart rate which decreases as the subject trains more.
I'd say your 88% of the max was a pretty well run half marathon, At the beginning it can be lower but after a couple of miles, you should hit both goal pace and goal HR (and of you did it right, your sprint at the end might not be that great), barring any hills of course.
Oh, you hardly reach your max heart rate in a marathon... You might get closer at the end of a 5K race.