Hi Mike - At the grand old age of 47, I can recall eating junk like that when I was in my 20's and I didn't put on much weight either. But after a while the body gives up under the strain and sure enough the pounds start piling on. You are doing a great job getting on to this before your eating patterns give you a bigger (literally) problem.
Don't expect changes overnight. Chances are you will feel worse before you feel better, but take it a day at a time and soon you will be feeling fab. I find planning ahead really important to ensure I don't find myself walking into McDonalds or the local pie shop when I'm hungry.