I did think of something else, it is an old saying that goes; "You get in shape to run, you don't run to get in shape". One of the most common things people I know that are close to my age give or take get to do is have knee and hip replacements. If having fun matters more to you than finishing the triathlon at a certain speed, then quit running the running part of it and just walk it, until at such a point in the future you get back down to the weight you were at when you were in shape in the marines. I know a 72 year old woman who runs all the time and does triathlons, and she is quite tall, but she is like a twig so there is no big stress on her knees or hip joints. If I try running anymore I do it on grass or sand, when I was young I used to run on pavement with bare feet and it did not bother me. The people who interjected about running styles and minimizing movement are hitting the nail on the head. I see overweight people trying to run all the time locally on paved routes and it makes me cringe thinking about what they are doing to their knees and hips. I wish they would switch to walking, cycling or swimming and concentrate on diet until they dropped off most of their bodyfat before they got into running.
I wish I could run like I did when I was young, I was in track in H.S. and did 10Ks and run/bike events after H.S., but heart problems took that away from me along with age. Once you are old and out of shape, it is a long road back to being able to do anything that a 20 or 30 year-old can. I have tried jogging and running over the last year but it simply does not feel good anymore. I used to be able to do a 7-minute mile pace or even a little better when I was young in H.S., and some sixty year old people can probably do things like that, but it is not going to be us big heavy people, we are just hurting ourselves. I love riding a cycle, and I love going for walks and waking up in the morning and many other things, so I don't have to be able to run to enjoy living.