Congrats on the 25 mile ride! I know what you mean about getting passed...I always think I'm riding pretty fast until I get passed like I'm standing still! I feel like increased speed is coming pretty slowly for me. I've increased my speed by about 0.5 mph on hilly routes and 2.0 mph on flat rides over the last 6 months. At any rate, getting out there and riding, especially on hills, will definitely help with speed in the long run.

I second the indoor trainer idea. I just picked up a magnetic trainer at an REI used gear sale and I can already tell that I've improved my spinning. For some reason, the trainer has made me realize that I have been mashing too much on hills. I don't know why I didn't realize this while I was actually on the hills! Anyway, now that I've practiced some really good spinning in the comfort of my living room, I am doing a much better job of pulling the pedals all the way around the pedal stroke when I'm going up hills. And amazingly enough, this increases my speed too!

Good luck! I'm sure you'll get there with the speed thing. And even if you don't, at least you're out there riding!