Gary Sinise would be perfect right now to play Pug. If they were to ever remake this, he'd be the first choice. (For FDR, they could use the OTHER definitive Roosevelt portrayer, Edward Hermann).
Mitchum looked far too old to play Pug, as well he looked really out of shape. Pug was supposed to be only in his late forties when the narrative begins (He turns 50 in the spring of either 1940 or 1941; I forget which), while Mitchum was in his mid sixties. As well, he was clearly out of shape and overweight. Pug was supposed to be in good shape, especially for his age. In one scene it was mentioned that he had the same pant size he had when he got married to Rhoda.
Other posters have mentioned that Pug was supposed to be short in the book. True, but that doesn't matter as much as how old the actor looks.
As well, Mitchum just didn't look like someone that Pamela could fall so hard for.
I think Kirk Douglas would have been a much better choice. He was good at playing military characters and he always had the dash (and good physical conditioning) to make Pug a more believeable lead. James Garner, at the time, was just the right age and could have pulled it off.
I thought of the perfect actor to have played Pug back in 83, rather than Robert Mitchum.....Robert Wagner.
He was right around the age Pug was supposed to have been in the novels, and you could imagine Pamela actually falling for him.
Another idea, and I know this sounds strange now, is Leslie Neilson. NOW, the thing to keep in mind is: back then, Neilson wasn't known for doing Airplane/Naked Gun style comedy. He was known as a serious, dramatic character actor.
Intersting thought about Leslie Nielsen as Pug, but it just doesn't click for me (probably due to his hair having gone totally gray/white at such a relatively young age). I could see him as an excellent alternative for the role of Palmer Kirby, but not Pug.