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.