If you're looking for structured training indoors using a trainer and a PM (power meter) or VirtualPower, TrainerRoad is tough to beat for $10/month. Using VirtualPower, as long as your trainer is supported and you are consistent with making your settings repeatable (tire PSI, # turns of roller on rear tire, and resistance level if your trainer has one) then doing an FTP test to use as a guide for your training will work very well, keeping in mind you shouldn't try to compare your VirtualPower numbers to those from a real PM.