I'm going through a somewhat similar point in my life, with an 8 hour a day sedentary job that I usually don't do much more than sit on my ass and take calls from people who really shouldn't be using iPads. The biggest issue is that it's 4pm-midnight, and I am not a morning person, usually punching my alarm clock for an hour before finally getting up to go to work. I generally eat something for breakfast (around 3pm), something for lunch (anywhere from 7pm-9pm as schedule dictates) and don't ever feel much like eating after I get home, especially after working out.
This is in stark contrast to my previous job, which had me on my feet the whole time, occasionally lifting 40 pound frozen soup boxes, and being surrounded by food, while having a schedule that ranged anywhere from 6am to 10pm as needed, not including going to college during that time period.
I recently started a stint at a gym, going 3-4 times a week (Monday through Thursday) after work, which is nice to have open since pretty much everything else is closed after midnight (and I can't even buy alcohol in my state after midnight, which is a sad travesty). While I may be mentally exhausted from dealing with stupid, I feel more physically energized after I get off from work and spend an hour doing a scattershot of about 20 minutes heavy walking on the treads (max incline at about 2.5-3 mph will cause you to sweat), and doing 20 reps of lower weight leg, back, arm and abdominal exercises, about 10 different machines in total. After 20 reps, I see if I can do 2-5 on double the weight. I've found that the seated leg press enables me to do a few (small, not full stretch) 390 lbs reps, which is something.
Prior to finishing high school, I was doing anywhere from 2-5 days a week of martial arts, which didn't cause me to lose weight due to my shitty eating habits, lack of self control (which is funny, considering how much of that was based in self control) and even more sedentary lifestyle. I still maintain my flexibility and dexterity from those points in time, but extra fat gets in the way of being completely able to move around as I used to.
In the two weeks since I've started, I've lost about two pounds, and feel a lot better overall; just need to keep this going and maybe I can keep seeing more results. As of my last week weigh in, I was 245, and still loving the ability to push myself physically again.
Any tips that people may have for someone attempting this? I'll read back through the thread when I have a bit more time, but it is work time now.