It sounds like you could lose a few more kilos but not many, that'd let you run faster but not much. Getting away from a plateau is very tricky, sooner or later the answer will come to you. A new training partner, a new routine, less weight, more miles, its very individual and can take a while to work out.
I have tested my heart rate on a treadmill on several occasions, it doesn't go above 182 and from 170 or so it gets harder and harder edging up a single beat at a time. I'm 42, so its about average if using the calc 220-age.