For the original op question, if you can get a smart trainer and sufferfest you will improve quickly. Agree with all the other advice on groups intervals etc

Remember that as you start working harder and harder on your rides you will have to throw in some rest periods. I used to increase intensity for 3-4 weeks, then take an easy week, then pick back up again and increase intensity, then do another rest week. Same thing applies in the gym. Otherwise you'll reach a certain point and just maintain...which is ok if that's as fast as you want to go, but if you want to keep improving that won't work. It also seemed to me that once you get a decent amount of fitness that you can definitely get faster by riding faster...ie: sitting in with a fast group will help you go faster.