I'm always tired. Even now - again tired. What can I do?

Hello to everyone,
How can I start? I'm 53, male, 5,11, 280 lb, divorced for 10 years, work 6 days a week, often have to stay overtime. Traveling to work and back takes quite a lot of time - every day around 2-3 hours. Most of the time I work outdoors - I'm a construction site supervisor, so winter and summer are problem, I get quite tired not only because of the job, but of the very high/low temperatures and humidity, as well.
I don't have much free time for exercise or working out, actually since high school I haven't practiced regularly any kind of sport. So I gained some extra pounds. I get exhausted easily, have troubles to sleep at night, sex is getting worse. Tired is now my second nature - in the morning, afternoon, evening. OK, I know I have to take a break. But even when I get some time off work it is difficult to relax - the situation of being with no job and nothing reasonable to do is horribly stressful, I guess my job is my life.
Yesterday a friend of mine, a very good friend from school, was again nagging: "Change your diet, stop the junk food. You have to work out." In college he was a basketball player and still continues to play and coach now. So he talks and talks: "If you don't have time to work out, at least get some vitamins or sort of." And he mentioned a few internet vitamin sites and recommended things like DHEA, fat burners and other stuff, that I haven't heard before, said how good they worked.
So what would you recommend me to take from these sites, the variety of stuff is huge.