This is really an interesting thread. I do think that perception of work can keep people from trying to exercise because any shortness of breath is perceived as a threat.

I _would_ use a HRM. I have a Polar and a Garmin (using the Garmin the most on the bike) and both work pretty well. I would look at two pieces of knowledge.

first there is the Rated Perceived Exertion:

http://my.clevelandclinic.org/servic...cise/rpe-scale

This article recommends that you work at the rate of 3 to 4.

The concept of Target Heart Rate will help you to know if you are working too hard. Here is the link to the American Heart Association on that:

http://www.heart.org/HEARTORG/Gettin...41_Article.jsp

The goal with aerobic activity is to sustain a heart rate that is 50-85% of your maximum heart rate. I can ride with a heart rate of 125-135 "forever". When my HR gets up to 140 (climbing a hill), I start to "feel it". But all of those numbers I just described for myself--they were not that way when I first started. I worked to get to this part. ALso because I am pretty fit, my heart rate comes down fast with rest (faster recovery).

So my point is this. Know your target heart rate zone. Strive to work with the lower to mid range of it. Notice how you feel, correlating heart rate to perceived exertion. Remind yourself that breathing faster, sweating, feeling your heart beat are all positive physiological responses to increased demand. They are not of themselves bad. However, dizziness, weakness, chest pain or shortness of breath that does not improve with 3-5 minutes rest are signs that your body is trying to tell you it needs help. Stay well hydrated and if you are riding for longer than an hour, consider taking a modest snack, but by far water is the most important.

I agree with Crankin's remarks about anxiety. Have a plan (the common safety measures) and have a construct to measure whether your body is tolerating the work you are asking of it. And then just trust your body and your bike.

And one last admonition. Make sure the old bike is tuned up and consider putting smooth tires on it unless you are doing dirt trails.

Stay well hydrated. Take your cell phone with you. Make sure your DH knows when you leave, roughly what you plan to ride and when you expect to be back. These are common safety measures.