"ooooo i'm a manager at walmart" doesn't sound as appealing as "I'm in the military".
I can't get a job in addition to Walmart, because I don't have a set schedule. I can't get an apartment with anyone, because my pay is shit and I don't want to risk the inevitable "Oh. I had to get gas. I can't pay for my part this month".
As for the physical demands, fuck it. If they let me in, they'll have a good idea as to whether or not I can do it. If I do get in and I get hurt, at least I'd be trying something that I've had in the back of my mind for a while. I actually FEEL like doing something in my life. It's only a matter of time that I find something enjoyable, and this might be it.
I'm going to actually make a serious contribution here, so bare with me.
From what I know of your previous employment, You obviously very much dislike working with customers. The problem with your current career situations however, are more troubled with problems of job commitment. If you stay at a job for less than, oh, say 1-2 years, new employers see it as a commitment problem.
I'm not saying that you DO have a commitment problem, I'm simply pointing out that the jobs you seem to land, always have a way of causing misery with you, ultimately ending in you quitting. I see it all time with people. It's not that they're crappy employees, it's that they can't seem to find the motivation to continue a job they dislike. Nor should you have to. But in today's world, an employer isn't going to look at a resume that says,
"McDonald's - 2 weeks: Reason: My manager hated me."
"Wendy's - 1.5 months: Reason: My Co-workers were assholes."
...and so on, and so forth. It isn't fair, but it's the way it works.
As for the army, it can bring many beneficial instances to your life, such as what you've said yourself in the original post. However from what I gather, it seem you're coming to the conclusion that, now correct me if I'm wrong, "I can't keep a job for whatever reason, I'm too poor for school, so my only two decisions are become a bum, or join the army." I hope that's a fair enough dumbing down... perhaps I'm off. But regardless, you should first look into perhaps a factory or manufacturing job. Try it out for a few months. Great pay, and you don't deal with customers. Maybe look into technical support or phone-related customer service.
A lot of places give the chance to further yourself in their company. The catch is it takes time and patience.
Anyways, I don't think you should resort to the army just yet. You're a smart guy, and if you work hard for a place you only very mildly dislike instead of hate for a good while, you may end up making enough money to start putting yourself through school. The army may just be a place to waste 3 years, and then come back to the same problem you're in now.