Don't worry about mileage on the trainer as much as you worry about time on the trainer. Also I find trainer time to be more efficient than outdoor riding time as far as training goes, since there is no coasting, stopping at lights, or distractions. I also find it easier to go harrder on the trainer than I do riding solo outside. Group rides and racing are a different story.

Trainer road virtual power is fine to train with as long as you have an accurate virtual FTP on TR to train by. Make sure you do a 20 minute test and do it right and the Virtual power on TR is a great tool. It doesn't mean you stop paying attention to HR though, Comparing your HR to your Power out put can tell you things about how your legs are the day in question.