What you describe is very common for athletes who start too fast and do not have sufficient aerobic base coupled with muscular endurance training. A low heart rate with high effort indicates local muscular fatigue. This is just a guess because we do not know what you have done for training. In what intensity zone is 150-160? Chances are good that you need to start slower. It is much easier to make up ground and places in the later stages of races when others are fading and you can still be strong than it is to go out too hard and then be the one who fades away at the end.
More Aerobic Base and establish your zones carefully. Add ME training on top of a good aerobic base and then pace yourself a bit more.