I'm a 36 year old long-time casual runner (15-30 miles per week, but with weeks off here and there). A couple months ago I started training for a marathon (my second - my first I ran 5 years ago in 3:49, though I seriously bonked and ran the last 6 miles in an hour, if that helps provide context for my question). I recently bought a heart rate monitor and have been noticing that my heart rate is fairly high at fairly slow paces. For example, I might be running at a 9:15 pace but have a heart rate at 155-160. I simply chalked this up to not being in very good shape and didn't pay it much attention.
Then yesterday, I was doing a 20 mile run. I did the entire run at 8:30 pace. For the last 5 miles, I felt very comfortable. I was chatting casually with a friend and didn't feel I was pushing myself at all. Then I looked at my heart rate several times and was shocked to see it was 175 or so the entire time. I found this really confusing because I was just running at a comfortable pace and in no way felt like I was running at anything near a max effort.
Do folks have any idea about what may have been happening? Is it possible I simply have a higher HR than people my age so 175 (which my watch says was in almost the top of "zone 5") isn't really that high for me? Part of my reason for asking is that I want to make sure I'm not training too hard (from what I understand 175 HR/Zone 5 for a long run would be training too hard). But on the other hand, if I feel totally fine and comfortable at a pace, I hate to let a strange heart rate dictate things.
I'm just very confused. Thanks for any advice!