Start walking at 4 mph. Then speed it up...and up....and up... By the time you get to 5mph, you'll find it's easier to jog than walk. If you can go faster get up into the 6mph range. If it's tough for you (but you are fit enough to try this), pump the pace up to 6.5mph. I bet you it will be easier to run at 6.5 than 6. And believe it or not, 7.5-8mph is even easier.
Let me clarify really quick. It's easier for your legs. You will find that your legs want to run fast. It may not be easier on your heart and lungs, but if you try to run at these faster speeds for just 30 seconds to a minute each time, you will find that it's easier for your whole body (legs, lungs, heart) to run at these faster speeds.
Just take it slow and work your way up. And (quick disclaimer) this is just from my anecdotal experience. You need to gauge what your body can do and consult with a doctor to be sure if it's safe for you to exercise. Remember, there is no shame in walking!