Hi I am training for London marathon (& possibly Milton Keynes). my heart rate is around 151-156bpm when training on a long run - this equates to around 70/75% for me. This weekend just gone I did a half marathon & my rate was about 156-168bpm (75-90%) as I ran quicker than in training. I am a slow runner at a pace of 11-12min mile. My question is to check running at 75-90% is normal or ok? Not a lot I can do about it but my breathing is ok, though I always get tired in training & races towards the last 3mi. I feel like I have no energy despite gels & a sports drink plus eating plenty of complex carbs a few days before. I don't want to slow down even more - my goal is sub 5:00 (or as close as poss). I have done a marathon before in 6 hours but had to walk a lot due to injury. Any polite suggestions welcomed. Many thanks.