Kennedy way too old

You don't? He was 38 and looked older. That is way old to be starting out on the rodeo circuit. It isn't as though he married young and had kids to support, so why the late start?

He was 4 years Older than Robert Mitchum. It was bad casting and took away from the realism of the film.

Probably they didn't want a much younger man, early 20's, because then Susan Hayward couldn't have played the wife. They could have gotten an actor a few years younger who had a more youthful appearance and manner.

I thought he was perfectly cast.

Kennedy did look his age and that was probably why Nicholas Ray refrained from showing him in close-ups most of the time. I'd say his biggest fault were his confused facial expressions that never seemed to quite fit the emotion he was supposed to be portraying in his few close-ups. That was probably an even bigger reason not to show his face up close a lot. But as to the OP's remark about him having a potbelly, just look at Mitchum and the gut he's sucking in throughout the film... and any film from the late 40's onward. But whereas Mitch had an overall good strong figure, Kennedy seemed very scrawny and unfit. His belly was definitely inferior to Bob's and it's not what made him a bad choice for the role.

I'm here, Mr. Man, I can not tell no lie and I'll be right here 'till the day I die