How do I get Jack to actually WALK...

When you 'walk' a cat, you are simply being led where they wish to go.
Nothing wrong with his prefering the stroller, after all, the whole idea is for him to get fresh air.

Spalding goes wherever he wants so we stay in the yard. Mister is very good at walking by my side and will happily walk down the sidewalk like a dog. When he stops for too long I click my tongue and say come on, and he'll start walking again.
I think it depends on the cat.

I call it playing follow-the-cat! Zissou will walk with you on the path sometimes, only when she feels like it. Sometimes she will make you run and sometimes she will lay down and not budge unless you pick her up and move her. If I'm trying to actually go somewhere with her I usually have to carry her, but mostly I just let her do whatever it is that cats do outside. You know, smell stuff, watch bugs, etc. She's also very scared of strangers outside (inside, in my apt she'll just walk right up and sit on their lap!) and cars and dogs and such. Usually she hides behind my legs like a little kid.

You can encourage them to walk more with you like a puppy by training, get him to go a little ways with you and then tell him good boy and give him treats and petting and such, and then try to get him to go a little further the next time, etc. As long as you still let him have his fun!