You should work at a rate that gets you slightly out of breath,
but allows you to just about hold a conversation. Or you can use
your pulse as a guide. To calculate this see the heart rate chart.

If you are new to exercise, work nearer your lower range
and as you get fitter and then more experienced, you can
work near the higher range.

You should note this calculation is just a guide. Some people
will be able to work very comfortably in a higher range.