Sometimes all you have to do to go faster is shift to a higher gear then bring your cadence back up to where it was before the shifted. You might find that you can now comfortably maintain this new combo. If not, then intervals, strengthening legs and all sorts of things suggested above.
I've many times been thinking I was at my max comfortable gear and pace. But it was just something I'd gotten used too. By trying something different I found out I could do more.