You may want to dropping $50 for a Timex Ironman heart rate monitor.
Figure out your target heart rate, maintain it (even if you have to walk) for 20+ min for each workout. You will find that, while maintaining the same heart rate, you will soon be going further and faster in the same amount of time. It will also help keep you from overtraining. You will find recovery much faster and you will be ready for your next workout in much less pain. 
They are excellent tools.
Now, I would also sprinkle in some sprints. Run a few hundred yards fast, walk/jog 2x the distance and repeat 3-5 times. This will help you get your wind faster.
Also, as mentioned already, mix your cardio up. Biking is EXCELLENT and easier on the joints. Also do some stairs. When you are sore, don't be too proud to just get out there and walk... work that lactic acid out!
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."In theory, there is no difference between theory and practice. But,
in practice, there is."
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"The difference between 'involvement' and 'commitment' is like
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was'committed'."
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