I would suggest your cardio fitness is poor so your current max is not your true max. Your legs are being limited by what your heart and lungs (cardio system) can sustain.
As mentioned above, slow down so your breathing is controlled and you sustain the same heart rate for the full 30mins. So, after 10mins you may need to radically slow down or walk to keep your heart rate down.
What you are trying to do is improve your fitness by slightly pushing yourself.
I'm in ballot for London. Also in a special ballot for Paris and I'll know if I'm doing Paris some the next week. I've registered interest for GNR next year for a fun weekend up there, I'm not planning on racing it after hearing my friends' reports from it.
Training for Stevenage marathon is on hold. Have a serious cold so I haven't run since Sunday and tomorrow's 20mile training run is cancelled unless I wake up a changed person.
So some swings and roundabouts for me at the moment.