I think people are right about you being under developed aerobically. I am in exactly the same boat, except I didn't have as longer time off as you.
My HR is slowly coming down on easier runs now after several months as I have been doing more miles, but it has taken longer than I thought it would.
I go more on perceived effort for easy runs as a result - if I can genuinely hold a conversation at that pace then it's easy, despite the HR.