Laura,
I used both CC and iTread to improve my speed.
I can keep up with runner settings at Grace's iTread workouts now. (Recovery/Steady State at about 6.5 miles/hour with intense bursts going up to 8 and on occasion even 8.5 higher.)
I run just twice a week, but I was already running for about two years when I started with iTread. At first I could only manage Jogger settings on iTread, and push a bit higher than those speeds for brief bursts. I would alternate between her Jogger and Runner settings, and gradually found I could increase the time I was spending at Runner levels. In about 3 months, if I remember correctly, I found to my surprise that I could keep up at the Runner level through the workout. For the first couple of months or so, I preceived very little progess - my ability to manitain higher speeds increased only the itsiest bit from workout to workout. Then .... whoosh, I saw dramatic progress.
I was 38 years old when I started iTread/CC, so if you are younger, or have not been running for as long as I was, your results could be different.
~* Vrinda *~