For those who are worried about overdoing it, they can get a heart monitor e.g. Polar brand. Such a device can be set to beep if your pulse is not within the desired range. It's not really necessary but may come in handy if you want to down load the data to your computer & track your progress.
There are also some Iphone & Android Apps that can be used with a bluetooth pulse transmitter.
Nowadays technology can make exercise more fun, but you still have to EXERCISE