New research* shows that running is no where near as bad for the joints as had previously been thought; and those excess pounds won't be there very long if you give it enough welly.
I had one non-functioning knee joint and a similarly decrepit ankle, but after using cycling for a couple of years to build some muscle, they have reacted extremely well to running - indeed, prior to starting regular running I wouldn't have made it anywhere near a Wainwright.
*Something I read in the Sunday Times a couple of weeks ago.