It sounds to me like your training is on track.
So on to speculating and random ideas…
What I recommend you look at next is maximizing nutrition. Work with a nutritionist or do a self analysis using a tool such as http://fitday.com or http://dietpower.com/. If you have a pop-tarts and pop diet then this could be it for you.
Glutamine – is magic for masters. Make sure you have some daily and especially post training.
Recovery – improving recovery will allow you to train harder and be faster. This doc has tips for improving recovery.
Plyometric training has been recently getting a lot of press for use with aging athletes. The theory is as we age we lose fast twitch muscle fibers faster than slow twitch. Plyometric training may maintain fast twitch muscle fibers.
Lynda