I'm really new to cycling, but from what I've read on the forums, interval training is the way to go in order to gain speed. I see you said you're doing intervals, so I'm not sure why you're speed isn't increasing.

What is a computrainer btw? Perhaps that's the reason you're not gaining speed. You might try doing intervals out on the road with your bike. Use landmarks and go as hard as you can between them, then rest, then repeat.

Like I said, I'm new, but this is info I picked up here on the boards. I'm interested in increasing my speed as well, but am trying to focus on just getting miles in at this point. I'm usually toast after about 7 miles so congrats at being able to ride for 50 miles!

I'm sure some other more experienced members will chime in as well.

Good luck!