Yep what Schmunk's said.
Running used to involve going running and not knowing how far you had gone other than by RPE and time ran. Occaisionly you would measure a course in the car or on a bike and use that as a test course. You effectively have the same situation with your turbo.
It isn't wrong but you may be able to train more scientifically and less subjectively. A HR monitor would at least allow you to train by heart rate and some would suggest this is the best way anyway. Get into your HR zone and push enough to stay within desired zone. Mentally quite tough.
Soemthing that might give you a distance equivalent could be considered. If you have Garmin 310XT or other simialr watch/gadget you can fit a cadence thing GSC10? and it will give you speed/distance from a spoke magnet and cadence from a crank arm magnet. If you have the watch the cadence sensor thing is quite cheap and worth getting. If you haven't got a suitable watch it will be a more expenisve purchase.
A cheap bike computer will do the same for distance/speed.
Note that neither are true road speed unless your turbo perfectly matches the resitance of the road/wind/air resistance. But they are a consistent point of reference for a given resitance.
M..eface