I have a Polar HRM and can't ride without it now. I use to be like you - no excess on the bike. But I was riding blind. Didn't know how far I could push it because I had no baseline.

Becareful with how you read your HRM. Your heart rate will drop as you get more fit. If you consistently ride up a hill at 180 bpm when you first get it then notice the same ride only requires 160 bpm several months later then you've improved. You can go faster uphill. Take the max heart rate test like every 3 months on a trainer if you can to see where you're at.