I use TrainerRoad and highly recommend it. You can "pay as you go" for only $10/month, which is a tremendous value. Subscribing gives you access to about 600 workouts and two dozen structured 8-10 week training plans. There are two things I like best. First, by taking the 8-minute or 20-minute test, the program establishes your FTP (power). Then all of those 600 workouts are customized to your level of fitness. Second, the real time feedback on my power, cadence, heart rate etc keeps me honest. I tend to be a bit of a slacker, but when the target says 125 and it shows that I'm only at 98 ... well, that's the info I need to put in the proper effort.