Share your heart rate?

My marathon pace that I use for training is between 85% and 90% of MaxHR. (Accurately measured and not Calculated)

I typically don't race with HR mostly because studies have shown that it tends to result in Sub-Optimal Performance. (On Race Day) But, I have a few friends that do race by HR and seem to contradict the studies. So, I might try it someday.

I usually race by pace and from my training, have a very good idea as to what my pace should be given the course and the weather. (Handicapping weather is definitely an art)

For workouts that include marathon pace miles at the end, I usually start out at about 85% to 88% of Max and work into 90% for at least the last half of the session. For example:

Where my plan calls for a 15 mile Medium/Long Run with 10 @ MP, I will usually do the first 5 miles with a target HR of about 85% but allow it to drift up a bit as I start working harder. Then for the final 5 miles, I will work up to 90% and hold it there for the remainder of the run. I am usually right on the ragged edge since my LT is right around 91% of Max.

Hope this helps

But your goal is to BQ, right?  If your HR 'tells' you to slow down and you finish strong in 3:16, what's the point?  Go by pace.

Note:  I'm part of the "wiser demographic" and I won't be wearing an HR monitor when I try to BQ in Philly a few weeks after you.

Does 7:25 equate to about a 3:14 Mary?  (1:32+5)X2 will roughly give you what  you should be able to do if you handle the nutrition right and have the endurance (long runs).  I don't know how you factor in the hills or how much more fit you are now than when you ran the half.  My heart rate averaged 173 (88% of my heart rate maximum) over a marathon last fall, surprisingly this time I did not see much cardiac drift -though I do plan for maybe 5-8 bpm increase later in the race as I dehydrate some.  I think many books will say something in the 75-85% is typical for a marathon-hence lots of variation only you can find out what you can do as %HRM in this race. 

Before the race, obviously do a run that is maybe 8-10 miles and try to run some of the miles toward the end at your goal pace 7:25.  That will let you know your correlation between pace and HR at goal pace.  In the race, I would try to keep my heart rate reasonably steady (though there obviously will be ups and downs due to the hills) and if you have a GPS check the pace on the flat portions in between when you settle down from the hills. Assume your heart rate will get at least 5-8bpm higher by the end it will happen so start at a slightly lower HR than you plan.  Don't even bother looking at HR during the first couple of miles, there are physiological reasons for it spiking a little high early on that are unrelated to the "excitement" so it will be quite a bit high early.

The pace is the objective not the heart rate, but the heart rate will give you some feedback as you go.

I think a lot of peoplle have pointed out that you should not rely too heavily on HR.  It is just one bit of data I monitor when I race.  Mostly it is how I feel and the pace I am going, the HR usually just confirms what I already know.  Race by feel, if you've trained properly you know where you are - frankly you don't even need a watch at that point.

It is quite common for me to not even glance at HR more than a couple of times throughout a race, mostly late in a marathon.  I do look at the data some after the race and then a few times prior to the next race of that distance particularly if I haven't raced for awhile.  I think my Garmin helps me learn and make the correlation between pace and the feel of the pace.  I don't hit tracks a lot so I find it very helpful in preparation.

Last year at Baltimore my average through the first 15 or so was around 173 (where it should be for ideal marathon pacing for me) which had me running in the 7:15 average range for splits.  Miles 16 - 20 are basically uphill and I averaged closer to 180 in that portion (more of my 1/2 marathon effort level) and then the last 6 it did not come down much because of the point in the race and general level of effort.  Ran a 3:14:58, so got my second BQ with a minute to spare!

But I did not pace off of it.  If I had, I would have felt obligated to back off on the hill portion, which I didn't do.