With my experience (that which is ever so little), I know that training on a treadmill is very different than outside.. Some differences I can think of:

• Pacing may be different since you can't "control" your speed like a treadmill
• Temperature (cold vs. the "perfect" temperature)
• Wind
• Humidity
• Terrain (asphalt, sidewalk, hills, etc)

Don't be discouraged if you can't get the same mileage out of an outdoor run as you can with a treadmill.  I just finished my first 5 miles EVER and I know there is no way I'll be able to complete that outdoors.. Still, go do it!  The only way you can truly know is if you get out there and run!

In the winter months, I tend to run about 70% on the treadmill and 30% outdoors. In the spring and summer, that ratio flip flops.  In my experience (which has only been a solid year), I find that the treadmill helps me in gaining speed.  I know that when I run outside, I run about 1 mm short of what I run on the treadmill. So if I'm running a 9:00 mm on the treadmill, I'll probably run about a 10:00 mm outside.  And this is all due to the same conditions that middie26 listed.

And I don't know about anyone else, but after running only on the treadmill and actually taking my run outside, you feel like you're working muscles that you didn't know existed. There's more of a need to stabilize yourself and push yourself through each stride. Also, the natural inclines in the road demand that you change your stride length, and speed at times.  But after a while, your body will strengthen those muscles needed to push you through an outdoor run.

Good luck! And congrats on completing the C25k program!

How did it work out for you? Let me guess- you did great, better than expected. My experience with treadmill is this: Gives you confidence and training when you can't get outside to run. Here is what I think:

Pros of Treadmill

You get to run and work out when you would not normally

Running faster on the treadmill will make you faster on the road for two reasons - yes it is easier but you can crank it up and down as much as you want and not worry about wear you are and how to get back to wear you started, You can try many things you would not want to outdoors, because it is harder to run outdoors than treadmill you won't push yourself as often.

Think of it this way- A baseball player takes batting practice from a machine - it is the reportitions that train him and then he takes it to the field against man which is much harder. It's like swinging the bat with a weight on it to strenghen is swing. I know when I run outside it is harder but at race time the adrenilen kicks in and before you know it you are getting better and better.

I have to disagree with all the other posters. I am a brand new runner as well. I do 99.9% of my training on the treadmill. My first outdoor was a Turkey Trot (Thanksgiving of course) that was suppose to be a 5k. After I signed up it turned out to be a 4 miler. But here is the good news...I walked more than I thought I would, but I beat my race pace that I had set on the treadmill. Some of it has to do with excitement I'm sure. So I'm glad that you are going for it. I look forward to you posting your results.

Set the incline to 2 or 3 percent.  Then running outside will be easier.  About a bizillion years ago I read 1 1/2% grade on a treadmill would feel like running on level ground outside, so never set the treadmill to less than 2% grade.  Don't know where the information originated, but it worked for me.

Running on the treadmill is much easier on the knees from the standpoing of side forces on the knees.  If you do most of your running on the treadill and then run on the edge of a crowned road, your knees may complain.

I had a buddy that did treadmill only and he had issues running downhill initailly since his treadmill had no negative incline.

Eric