Been off the site for awhile so pardon a late two cents... Bear with me, and I'll try to get to my point as quick as possible!
I've watched a lot of co-worker who used to work construction and like me, changed careers for their health, gain a lot of weight in the first few years. To me, you have to change your eating habits in relation to the calories you burn.
I'm a 6'1'' former "Salamander" (Commercial Steam Pipe Insulator) who used to run up and down ladders, work the sides of chemical tanks on pole scaffolds, or crawl around under floors in four foot square tunnels repairing insulation after pipe fitters did their repairs. Needless to say this job burned a lot of calories and I stayed somewhere around 170-180lbs. Heck, anything more, I probably wouldn't have fit in some of those tunnels. I quit in 2001 after my second knee surgery and a bout of what was later diagnosed as chronic asthma.
I don't exactly "pig out" when I eat, but I never changed my eating habits when I quit construction and IMMEDIATELY gained 20-30 pounds to around the 200lb mark. I probably walk 2-3 miles every day at the store if those pedometers are any kind of accurate but my metabolism dropped from not having to pull my weight up and down ladders and not having to deal with the outside elements or sweating off pounds when working in a hot boiler room for 8 hours at a time.
Last year I got this diverticulosis thing in my guts and it slowed me down even more for 6 months and I went up to 220 lbs.

Now being 6'1" and 220lbs doesn't SOUND huge, but that's a 30% increase over what I'd been most of my life and those extra 50 pounds were proving he// on my knees! That's when I finally realized I was headed for trouble. I had to change my diet anyway for the stomach problems and I started back to the gym trying to get into shape to be a bone marrow donor for my brother. (they say its almost as hard on the donor as it is on the patient for a couple of days. Don't know it thats true, but I wasn't taking any chances!) I'm back down in the 190s and the knees feel better again!
My point is, when life forces change on you, your appetite doesn't immediately recalibrate, and you don't always notice the change until you're WAY off your mark. I thought those first 20-30 pounds were just "turning 40" finally catching up to me. I was still very "active" but walking isn't the same as climbing a ladder and working in a nice air conditioned building isn't the same as being on a rooftop in the summer or freezing your but off outside in the winter. Our bodies are very efficient machines and they turn off systems to conserve resources when they aren't used. A combination of convenient excuses and a lack of understanding of this can put you down in a hurry. My 300 pound laborer friend who was so strong he moved credenza file cabinets LOADED, by himself, died 3 years later after a back injury retired him. He was well over 400 lbs when he died. One of my old shop stewards from a prior job was an all district linebacker in school is one of those "Walmart Express" riders you're talking about. Asking him how it happened he still doesn't really have a clue! My wife and my brother are both fighting health issues but at least see the warning signals and are trying to change things now!
Awareness and good sound advise at the start are the keys. If you wait until you KNOW you have a problem, you've got a longer fight ahead of you!...just my two cents...