I totally understand where you're coming from. I'm in accounting and finance so I'm all too familiar with butt sitting all day every day lol. When lunch time hits I try to step out for a quick 10-15 minute walk, granting the weather permits. When I get home, I grab myself a quick light meal for some teompary fuel and I hop on my bike trainer or go out for a run. Again, all these little things that you can do throughout your day can add up. 10-15 minutes of something here and there and you top it off with at least a half hr to 45 min of your treadmill session next thing you know you've accumulated at least an hr of activity, which is pretty much average amount of time someone spends at the gym.

What it ultimately comes down to is a balanced diet. I always it's 20% workout 80% nutrition. You don't have to deprive yourself of food you love to eat, just take things in moderation and in proper timing. That way you won't have work any harder to burn anything off.