This is interesting. I'm curious as to what kind of responses you get to this question. I would definitely think you would be faster on the treadmill versus outdoors since you have to keep up with the belt. Hmm...let's see about this. Len? Jay?

I have my own feelings about this, because I find it harder to run on the treadmill than outside.  The reason, for me, is that I have to work harder to maintain an even pace and to stay on the belt.  It's a much more "controlled" run.  As a result I usually go a little slower inside.  On the road, if I misstep or my mind wanders and I slow down for a hundred feet or so, it doesn't matter.  The road's not moving.  The soreness you're having is most likely due to running considerably faster outside.  15 minutes faster over 8 miles is a lot!  That's almost 2 minutes per mile.  You should be sore.  I guess that'll teach you to run outside.    I don't think the shoes matter much.

Len

If you hadn't run on hills and uneven surfaces before, your muscles and joints would have gotten a little different workout than you're used to. Plus the road is a harder surface, so it's not surprising at all to me that you're sore!

Like Len, I am slower on a treadmill and I find it more difficult to run on. I never really thought about the effort and concentration it takes to keep up with and stay centered on the belt; he has a good point there.  I recognize their value as training aids and I own a low-end one for when it's too cold outside, but they are in no way the same as running on the actual ground with actual wind resistance in the actual environment with actual hills (both up AND down) and bumps and dips and sights and sounds and smells!

I never have quite understood it either, but there is just something different about running on a treadmill versus running outdoors. I ran on a treadmill religiously every day for a year. Then my husband wanted to start running and we started running outside. I thought it was going to be the same and found that even running the same amount of time I was really sore after running outside. I found it quite annoying because I felt like I had been wasting my time on the treadmill and haven't run on it since.

However, I have a friend who trained for a half marathon entirely on a treadmill and did just fine.

so I thought it was me

It still may be, but who knows.

I am with Len,

It has started getting cold here in VA, I decided to run on the treadmill for my run today.  I am 6'3" and have a long stride so after I stepped off the belt 4 or 5 times, I donned my gloves and beanie and finished my run outside.  I guess it all comes down to preference.  I tend to let my mind wander when I run..... I've seen people fall of the treadmill, I don't want that to be me!  It could be bruising to the body as well as the ego!

Lachupa, it's certainly possible to train for a race just on a treadmill.  But I think you have to be familiar with running on the road too.  A couple Olympics ago, one of the U.S. women trained for the U.S. Olympic Trials almost entirely on a treadmill - 20 mile runs and all.

Len