Heart rate and fitness

I'm trying to improve my fitness and have recently invested in a heart rate monitor. I'm a cycle commuter and most days I cycle twice a day for 30-40 minutes at an average speed of 20km/hr (on a slightly hilly route in traffic with a backpack weighing a few kg). I've been doing this for 4 years now and never seem to get any faster or fitter. I get puffed every time I walk up a hill. I thought a heart rate monitor would be a good way to motivate myself to push myself a little harder to improve my fitness, but from the results it seems I am already pushing hard enough. I'm 37 with a resting heart rate of 60/min. My heart rate when I ride sits around 145 when I'm stopping and starting for traffic, 170 when I'm riding with a comfortable puff on an uninterrupted stretch (about half the total distance), and went above 190/min for brief periods uphill. So according to some formulae, I am exercising at 80-110% of my max heart rate for 30-40 mins, 8-10 times per week and have been doing so for years.

I am overweight (BMI 29) and I am working on this separately through diet.
But why am I not fitter than I am?