Best tactic to make a lifestyle change?

I have a lifestyle where most of my day is spent sitting in a cube in front of a computer. I eat lunch at my desk, mostly because I have too many things to do to take a lunch break. I’m usually at work until after 7 pm each night. As a result, my eating habits are lousy. Not in the sense that I eat a lot of junk, but that I don’t eat regular meals at regular times. I usually skip breakfast, eat a decent lunch, and have soup or a sandwich for dinner, or some sort of leftovers.

I don’t get any exercise at all, and have noticed that I cannot walk for any distance without something hurting. I cannot see myself exercising in the morning, and I’m not inclined do so at night. The gym at work is packed during lunch time.

My eating habits aren’t horrendous and I don’t overeat; I can’t remember the last time I ate fast food. I try to avoid processed foods as much as possible, and don’t each much in the way of meat. I like most things, except for fish, which I don’t dislike.

I want to change my eating and exercise habits. What approach should I take to ensure that I will stick with the program?