Here are a few thoughts or you:
1. Get a heart monitor and learn how to train in different zones. Here are three great links to help you understand this stuff:
http://www.trifuel.com/triathlon/tri...ate-000574.php
http://www.duathlon.com/articles/1460
http://www.lifetimefitness.com/modul...ning_chart.pdf
The last link is to a great zone chart from Lifetime Fitness
2. Think about using an eliptical trainer. No/low impact and all the benefits of running, especially with your concerns. I hate running and love these things. I actually use an Arc trainer which means it just doesn't have the arm portion and you can use handles to simulate climbing as part of your workout. Plus, they read your heart monitor strap.
3. Think about a spin class. You can be a complete, out of shape novice and just work at your own tension level on the bike. You can really build a great fintness level over a few months.
That's my two cents worth!