The space that I have to work with is pretty limited and it needs to also work as a pass through (the stairs to the finished attic originate in my new office.) The Tread Desk (www.treaddesk.com) "Tread" at 58"x24" is the smallest motorized treadmill that I've found. The fact that it's supposedly designed for long term walking is also a plus. It's just that I haven't seen anyone's review on this except for the owner of the company (and I'm sure he'd admit that he's not unbiased.)
Not having arms shouldn't be a problem for me as I'm planning for a new desk in the new office anyway so I'll just go with a tallish standing desk.
I just found this page: http://www.drpribut.com/sports/walkingshoe_fit.html
where he says 6 to 9 months for walking shoes. He isn't expecting 10 miles a day of walking, though. But then again we are walking slowly, which reduces the impact on the shoe. Unless I find out other information, I think I'll aim for alternating two pairs of shoes, and keeping them 9 months.
This thread was a big help in making my choice of treadmill desks. I've gone with the Tread Desk tread. I already have a standing desk I've cobbled together out of a two-level Anthro cart. I'll post some pictures when my tread gets here.