When I got my first office job, a coworker told me about "the spread." Take a look at your coworkers and see how many of them have smooshed, spread out rear ends, sometimes twice as wide as the rest of their bodies. My coworker attributed this to them sitting all day. This has always been great motivation for me to get up outta my chair! 8)
I have to agree with James in that working in an office won't allow as much activity as working on a farm does, just because of the nature of the jobs. There are, however, some great ideas posted for staying active while at work.
I use the time at my desk to work on mobility exercises/stretches. For instance, I have plantar faciitis and I have a golf ball that I roll around under my feet. I also try to stay focused on my overall posture and body tension. Like many office workers, I tend to hunch my shoulders so I try to keep that in check. I also clench my jaw habitually, but I have a reminder to help me loosen up. Of course, if I wasn't stuck at my desk, most of my flexibility/tension issues wouldn't exist...8)
I try to get out of the office at least once a day and walk around outside. It takes maybe 10 minutes total to get downstairs, lap the two buildings that make up our office "park," and get back to my desk. I also eat my lunch outside when it's feasible.
As others have mentioned, I make an effort to take breaks where I'm getting up and walking around, like filling my water bottle or talking to coworkers.
I think you're starting off with the right attitude: you have to be aware of the lack of activity that's common in an office and make an effort to up your own personal activity.