Finally, after years of trying, I've managed my first proper interval session outside. I have no problem with intervals on a treadmill, but I can never manage to speed up much outside. I've tried 4 x 4 intervals, and would never manage to finish the first interval. For once, I succeeded. It wasn't the most impressive - just 1 minute of fast running and 2 minutes of slow running x 10, but I'm finally on my way. So it looks like the key to outside intervals for me is just to take it easy, then build up to longer intervals.
How about the rest of you? Are you able to run outside intervals, and do you have any tips?