The entire food system is fucked, and it takes Herculean strength to overcome it.
Potatoes you don't want to go overboard on, but as carbs go they're among the better ones -- better than bread -- because their glycemic index is lower. And with the skins on they have nontrivial vitamin/mineral content.
If you did a little more cooking at home, particularly with beans (not in sugary ketchup, just beans with butter and spices -- salt, pepper, cumin, say), then you'd be a bit better, on account of the fiber. Add frozen or, failing that, canned vegetables, and you're almost actually good.
Often you can find canned chilli that's nutritionally pretty good; just make sure it doesn't have sugar; they put sugar in too much stuff.
Getting more of your calories from nuts is also a good option. Surely you can get a jar of peanuts? A large handful is 300 kcal. A couple of those spaced over the day can fuel you quite well with a slow burn.
The car-deprndrnce of the country is an issue; you probably would not naturally walk far enough. If there's some way to schedule in about two miles a day -- walk to the gas station for a black coffee? -- then that'd help to maintain some minimal baseline levels of activity.
Stick to the calorie count and maintain some (frankly modest) level of light exercise and you can maintain your health in a fairly wide range of circumstances.
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I get what you mean though. There's a real class difference. You go to higher class places and there are plenty of salad places around. You go to low class places and there's fast food junk. The latter isn't even much cheaper. It's enough to make you start wondering if people are in the classes they are because of the standards they have. That's not a comfortable thought, but, when you look at how all the bankers seem to also have good gym routines, you really do start to wonder.
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You don't need to do anything superhuman though. You don't need to even summon the willpower to hit the elliptical machine every day. If you just think about what you're eating, count calories, reduce sugar and fast carbs, and walk a bit, you'll do pretty well.
For the customer, perhaps. Higher margin, though, so the business can survive with fewer customers.
I think it is pretty clear that "high class" places attract more restaurant traffic. Case in point: The town I grew up in, of 1,000 people but a town of the rich and famous (you're probably familiar with quite a few of them), there are 15 restaurants (was 16, but one recently burnt down). The town where I live now, which is the next one up the road but more of a working man's town, has 3,000 people yet only seven restaurants.
Last time I was in McD the price of a salad (even without a chicken!) was close to the cost of BigMac.
But the most amusing is what I opened mcdonalds.com and there is no salads in the menu at all.