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That has got to be near or above your Lactate Threshold even if you are in awesome shape.
Yup -- and that is the point of a kick-ass marathon. If we define LT as the pace at which lactic acid is steady at a tolerable, yet high, level (4mmol - ?), then one should be able do that for an hour when in marathon shape. So, the final hour of a marathon would see that kind of HR.
The target HR for me is to hit 170 over the final 6-8 miles. I'm most certainly not there yet. I've done 10 miles at 165-167, and it was pretty challenging. I do a marathon-pace run every three weeks, and am building them up to 1:45 in duration (this is recommended in the Daniels book).