Running is a great workout, but it is very unforgiving on the knees and the lower legs. I was told by a DR when I had surgery on my knee that it was better to walk at a steady controlled pace then to run where your feet are pounding the pavement cause your trying to keep going.
I took his suggestion and walked 2.5 miles 3 days a week for about 1.5 months then got bored and started jogging alittle while staying in a controlled pace where I was going heel to toe. I started in May, in Sept I ran a 5K in 30mins. For the record I was 285lbs when I started and was down to 235lbs in Sept. Unfortunitly back to 240 but starting again now that its nice out.