For a newbie, just focus on running long and slow, and speedy recovery after each run? Since I dont have any specific target time for my first 50miles, 10hours would make me the happiest man on the planet, but anything below 15 hours cut off is perfectly fine for me
I have not use my HRM for a while too, just wear it last week to gauge my own effort, for sort of "calibration" on how you preceive your own effort