Nan;

I have a few words for you... Do it!  whatever it is, just do it!  Get your body moving - your body might groan, but it will thank you.

The C25K is an excellent idea. 

My opinion though.. If you are gonna do this - Do it properly. 

Eat Right!  Fresh Fruits, Vegetables and yogurt (I like the Greek Yogurts) are your friends!  I eat 5-6 times per day.  Yogurt - Fruit - Tuna Sandwich - Fruit - Big Salad topped with hard boiled eggs, grilled chicken or pork, sometimes a low cal iced treat like Italian Ice or fat free klondike bar.  

If you follow C25K and eat properly.  You will see that weight fall right off your body.  You knees will feel better, your back will feel better.  You can do it!

In November 2010, I was 236 lbs and was occasionally active.  My favorite treat was 2-3 bowls of cereal right before bed.  One day I woke up and said, "No More".  I got up the next morning and ran (not very far) then walked for a bit.  I have not stopped and my weight is down to 201.  I have had 3 knee surgeries so I know about sore knees.  My knees are thanking me for the exercise.  I have zero knee issues. 

I did it!  You can Do it!  So get out there and do it!