It's all been said Ray, those long slow miles in particular are essential. My fastest period on a bike co-coincided with a long 3 week tour a month or so before. Off the back of that and a bit more training, I managed 20mph over 30 minutes, which is still modest by club tt standards but was all I needed to do to scratch the same itch you seem to have.
15mph is still good though, mine is pretty similar of between 14 and 16 and without spending much more time on the bike and structuring my time, that's where it will stay.
15mph is still good though, mine is pretty similar of between 14 and 16 and without spending much more time on the bike and structuring my time, that's where it will stay.