I am your age. I really don't figure I am warmed up unless I am over 130. I can cruise easily from 140-150. Over 160 is a bit intense. Over 170 is pretty intense. I generally use my heart rate monitor for indoor spin bike sessions to give me feedback on intensity. On the road, I have plenty of other things to keep me amused so I don't use a monitor then.

As others have said, the 220-age is not really that helpful. It is just a starting place. If you want to know your max heart rate, you can ride just as hard as you can and then push harder and see what you get. Once you have done it a number of times, you should have a good feel for your maximum rate. But it isn't really necessary.