After you've grown accustom to standing, you'll be ready to make the leap to the treadmill. Mine is just shy of 5 feet long, which meant I had to clear out some room in my home office. You'll obviously need a flat surface to place the tread upon. If your workspace is carpeted, make sure that the tread you're looking to buy works on such a surface; if it doesn't, you may need a treadmill mat to place underneath the tread itself.
Make your choice
If you're handy with power tools, you may be able to build a desk, buy a used treadmill, and save yourself serious money. Sites like Ikea Hacker, eHow, and others have detailed instructions for homegrown solutions.
If you order a ready-made solution, you needn't fear much setup effort; treadmills for desks typically ship pre-assembled. You just plug them in and start walking.
Keep in mind that treadmills built for desk use will have key features that a home-built solution will lack. Chief among those is detached controls. With my TreadDesk setup, the control panel (for adjusting speed, starting and stopping the tread, and tracking my statistics) sits on my desk surface, adjacent my keyboard. If your home setup obstructs your treadmill's buttons or display, it could quickly become frustrating. You need to stop the treadmill numerous times each day--when the doorbell rings, or nature calls, or hunger strikes--and fumbling for the controls to shut the tread off (or get it started again) is a no-go. Make sure the buttons are always within easy reach.
Get ready to re-learn multitasking
To start with, you will want to set your walking speed no faster than one mile per hour. Seems slow, right? But don't forget that you could be walking for your entire workday, so you're getting ready for a marathon, not a sprint. Even more importantly, of course, you're not just walking for all those hours at a time--you're working, too! At one mile per hour, it's still pretty easy to type and conduct phone calls at the same time. If you double that speed, your typing will likely suffer.
If you peg your speed at one mile per hour and walk four hours per day while you work, you'll cross your hundredth mile in just five weeks. I'm now walking around 1.1 or 1.2 miles per hour for eight hours a day. It's not just hundreds of miles--it's thousands of burned calories, too. Each 3,500 surplus calories you burn is equivalent to about one pound, so you might want to start saving up cash for some new pants, too!