Posted: 23/07/2015 at 18:47
I use the HRM for marathon training and for the marathon. My HR goes from 75% of max for the first few miles to 85 / 86% of max for the last few miles, and it repeats it self in most of the marathons I have ran. Averaging about 82 / 83 % of max. You say your max "appears to be 190 " how did you come to that figure ?. Without knowing what your max is the training that you do will be in error.
One further point
Looking at your times for 5k and half marathon, you do need to do some aerobic training ie long slow runs.
As the others have already written and I agree with, train, train, and train and then work out what you can expect to run the marathon in.
Hope this helps in any way yours Mok