If by "too fast" you mean you are setting too fast a pace for your run segments, that's very possible. A lot of people push themselves to run too fast a pace when they don't yet have the fitness to sustain that pace.
If by "too fast" you mean moving on with the C25K program without repeating more weeks before this week, it's possible, but I'd look at slowing your jogging/running pace first. :-)
Are you tracking calories? I just signed-up for the livestrong site. So far I like their online calorie tracker the best, better than fitday or caloriecount.about.com. The livestrong site has a large database of foods from TJ's which is my primary grocery store. However, I haven't yet tried their recipe function.
I am running as slow as I possibly can without dropping back to walking. I've already gone through the "running too fast" phase and kicked the habit. One of the reasons I prefer a treadmill is so that I don't run too fast.
From my "Hey, it worked for me" file: I set myself heartrate targets.
With me, I have two "heartrate running" workouts. Easy: Run at a reasonable pace (I tend to use 90s-120s/400m, depending on how fit I am that day). Switch to walking at 160/165/170 bpm, run again at 130 or 140 bpm. Harder: Run a distance at 90% pace (eg 200m, 400m). Start again when heart rate is below to 119 bpm. (This is derived from http://www.brianmac.co.uk/sprints/tp400p1.htm. His paces are based on 100m time.) I then repeat until the recovery period doubles the initial one.
I sense my heart probably revs a bit high so you might want to set lower targets. This kind of heartrate target is very satisfying for me, because I've decided beforehand when I've "earned" a break, so it's serving a similar goal to your w25k. I also use heartrate cutouts on some weights workouts.
HR monitoring itself is quite cool, because I get to see my heartrate recovery curve (which has plateaus in the 150s, 120s, and 100s, and just burns through the 140s. Also, typically, my heartrate blips 5 bpm higher in the 20s after stopping exercise. Kind of makes sense that heartrate is a trailing indicator of oxygen debt, I guess.)
What I'd suggest is setting a heartrate cut-out on the longer runs, and perhaps going to fortnightly step-ups. Might be an idea to experiment with something like tabata dumbell presses or squats in there as well, just to provide a little variety. It might be that your body isn't responding to the training stimulus as dramatically as it was.