Start out slow. Thirty minutes at 3.5, three times a week. It may hurt initally, but stick with it,don't quit if you get muscle soreness, because once you get through that you'll NEVER have to do that again. You should add some very light weight lifting for your legs to build strength as well as endurance. It will help you increase your speed over time. Schedule out the next four months in your work out routine.
For example, every month, after my cycle is over, I increase the intensity of my exercise by a little bit. Thats the time that your body is at its most agile and best time for changes in exercise.
Hope that helped!