Ack, same with me! I'm so glad I read this. I thought I was the only one! I finished my C25K training a few weeks ago, all on a treadmill, and I was doing great. I could run the entire 5k with just one little stop in the middle to catch my breath and grab a sip of water for 20 seconds. Then I moved my runs outside. Almost killed me! A lot of my friends have been running for years, and they all told me how much better outside runs were, and how I'd hate the treadmill. I'm the opposite. Outside runs are hard! As mentioned above, you need to use your legs to push off unlike a treadmill. And the terrain has hills. And there's wind. And it's hard to monitor your pace. I also loved the treadmill because I'd watch all my stats and do math the entire time, constantly figuring out time in, time remaining, percentages, etc. Helped pass the time.
I've been running outside now for about three weeks. I found that I had to slow my pace down considerably from where it was on the treadmill (was running around 6 min/km on treadmill, now 7 to 7:30 min/km outside). I also quit trying to run without stopping, and I'm doing intervals of 6 min runs and 1 min walks. I accepted the fact that I can't compare my treadmill runs to my outside runs, and to just pick a place to start over from. The slower pace helps a lot!
I'm hoping I learn to love running outside, and that I'll complain about the treadmill when it's time to run inside again later in the fall.