Possible BR
I admit to not having a working HRM so go by my perceived effort.
I find when I went by the HRM, I am meant to go no faster than walking for what was then described as fat burning range (60 - 70%). Having said that when I first got a HRM about 7 years ago, I went on the 220 rule for max HR. Also my HR would shoot up for the slightest effort. This made me downhearted so haven't used the HRM since.
I think it is also possible that I am not trying hard enough on the speed stuff because I have too many perceived obstacles, e.g. roads to cross, people in the way, cars to avoid etc
Once I get the logistics sorted out by saying go down to a track, I will be able to try out the speed sessions properly.