Again... thanks for all the responses. Y'all have given me quite a bit to digest.
I should have given a touch more info...
I am not new to biking, but 2015 was my one and only year over 1000 mi. 14 and 16 were in the high hundreds and other than a small triathlon last June, I've only just started getting serious again. I really just figured that as hard as I felt I was going, it would show up in my heart rate