Recently, some things have come to light about the difference between walking on a treadmill in a gym and walking outside on a sidewalk that I thought I would share because I found them quite interesting.
Back in early 1999, soon after I'd joined a gym for the first time, I was working out 5 days a week, which consisted of walking on a treadmill plus some weight training. I didn't know then that I wasn't supposed to be doing the same workout every day and, besides, only the treadmills had TVs on them and that made it easier to get used to exercising. (grin)
Even after I was well accustomed to walking on a treadmill, I could not maintain much more than 2.9 MPH and even that would usually be limited to sprints of 10 minutes or so mixed in with other speeds. I also could not handle walking on an incline. Every time I would try to set the incline more than maybe a 1, I would have ankle pain (not usually while walking but later).
Fast forward to 2004...I rarely ever get on a treadmill anymore. I've branched out my cardio workout to cardio and toning workouts on DVD at home, aerobics classes, recumbent bike and occasionally the elliptical and stair stepper in the gym, and powerwalks with short jogging sprints outside. About the only time I get on a treadmill now is once in a while as a cooldown from some other cardio activity.
On a recent visit to the gym I, for some reason, decided to do my cardio on a treadmill. After an ample warmup at lower speeds, I decided to go up to 2.9 and see how long I could handle it. Not having been on a treadmill at high speed in a LONG time, I figured I wouldn't last long. 30 or 40 minutes later, I was quite surprised to still be walking at that pace. I brought the pace down a bit and then tried incline...didn't get ridiculous with it cuz I didn't want to risk ankle pain, but took it up to 5 I think. Walked that way for 10 minutes or so and had zero ankle pain later that day or next day.
SO...the end result of all that yapping is that it appears that all the walking I've been doing outside in the "real world" has better equipped my body to be able to handle higher speeds AND inclines in the artificial turf of the gym. Pretty cool!

P.S. to those who don't know: I'm only 4'6" so my speed on the treadmill is pathetically slow because I have short, stubby legs! hehe