Ive been reading up a lot this week, after taking some advice from a tri-athlete.
Isaac - the speed of your heart rate will be a good indication of the type of training you are giving your body.
People measure heart rates because it's easier than measuring how much oxygen your body is using.
Your age will be a determining factor in setting the parameters. For example, being about 10 years older (I think) than you, I will find my aerobic training zone about 10BPM below yours.
This chart looks like not a bad indication
http://www.ginmiller.com/gmf06/articles/target_heart_rate/heart_rate_chart.html
If you want aerobic exercise (rather than anaerobic) I would suggest a lower gear to get your BPM around 150 BPM.