90% peaks in a half-marathon race is fine, but 90% in training will leave you too tired to train effectively in your other sessions. For the marathon you won't be able to sustain 90% for very long, you should be looking to stay below 85%.
PS There's something wrong in your figures as 156bpm (75%) gives a max HR of 210 bpm, but 168bpm (90%) gives a max HR of 187?
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