I'm 68 yo and have done 6 marathons recently (since I turned 65) lots of half marathons, injury free. Kind of new to triathlon, only done 3 olys and a few sprints, attempting my first HIM in Haines City next Sunday. When I started running long, I started getting knee pain, so I reluctantly went to the Jeff Galloway method, alternating running and walking. Do okay for a geezr, PR on marathons is 4:39 - nothing to write home about but I got a podium with it, due to the few competitors in my AG. I would strongly suggest you try run/walk and gradually increase the run to walk ratio. You just might surprise yourself.