Make sure you take it in for tune ups, cables stretch,etc. and things can get out of wack(also if you are putting it up against other items in storage or in a car you could be moving the rear derailleur around without knowing it)

On the speed, I would say train more. 10-15 mile rides aren't a ton and your speeds for the short distance seem pretty good for such little training being put into the bike.

I would be riding a lot more and focusing on event specific intervals to help you reach your goals.