Stemming from the discussion on Martha’s blog about getting a new mattress vs. a new treadmill, I thought I might explain why (for me at least) having a treadmill to do your running on is significantly better than not having one (or having one that’s about to break).
Basically, it all boils down to a scientific approach to your fitness plan. If you go out running on the street, your speed will vary (consciously or subconsciously) based on all kinds of factors on any given day. You may be more tired one day than another, etc., so it would be hard to consistently (over a week / month, whatever) define what you did when you worked out, because it would be variable and difficult to measure.
On the treadmill, on the other hand, if you set it to X MPH and go for 30 minutes, you know that you were going that speed the whole time, by virtue of the fact that you remained on the treadmill.
That’s actually one of the problems with the one we have now, I think the motor is dying down, so it feels like sometimes when you step down it slows it up for a moment, and then when you step up it feels like it speeds back up to try and catch up. I haven’t really looked at it too closely, but after having used the ones at the gym pretty much every day for the last few weeks, it’s much easier to see the faults in the one in our garage.