I agree definitly with terrymorris on the training aspects. To increase your speed you have to ride faster, i.e. tempo rides. You will get to where you can do tempo longer and longer. Lactic threshold intervals are great for improving aerobic endurance. Racing I do those two days vs one day per week but I limit my intensity the next day after each session.

It sounds like you have a good base under you with the century training. A little bit of work and you will be there. My Advice on LT and other intervals though is to start them shorter, like 4 - 6 minutes then build up your time and then intensity. I started doing LT for 5 minutes, now I am doing 12 minute intervals x 5, with 3 - 5 minutes inbetween but I have been doing it for some time.