I also do interval running on the treadmill. The days I think I will try steady state running, after 10 minutes I am watching the clock and ready to scream with boredom. Anyhow, I seem to be able to go longer when I listen to music than when I watch tv. The longest I have gone is 45 minutes.
As far as speed, you have to find a speed where you know you can complete the running time you want. I am at 5 mph for this. Some days I go to 5.2. I can run a 12 minute mile at 5.0 and last for about 3 miles. For me, this is excellent, and I feel great after. On my intervals, my fastest is between 6 and 6.5 mph. I use the talk test to judge my intensity. I use "row, row, row your boat" for my test. I got this from one of Covert Bailey's books. However, if I run outside, all this goes out the window, because I have a hard time running at a good pace to last long.
Sorry this is so long. Basically, find a pace you know you can go steadily. Melissa