I'd say your performance is just fine. Why are you concerned about it? Are you interested in racing? Why only ride on that one mile? You don't need a heart rate monitor unless you want to really seriously train to ride fast for some reason.

When I was 18, at the end of my first summer of cycling, I rode100 miles in 6.5 hours on a 10-speed road bike, solo. And I'm essentially talentless. You could probably pick up a bike like that for $25 today. So you want a goal, there it is. They didn't have HR monitors or clipless pedals back then. Tennis shoes and toe clips.