^this, plus I'd also say that you need to vary the type of runs and the pace at which you do them, rather than just running in the same way every time.
When you've done your half marathons, have you followed a training plan, or just run progressively further, at the same pace?
In a training plan, a typical week is usually built around:
One long slow run (so that you can easily maintain a conversation)
One tempo run (comfortably hard, but not more than 40 minutes, ish)
One interval session (pushing yourself hard)
with recovery runs, cycles or swims (very, very slow) on a couple of the days in between.