You can not compare mph on trainer and mph on road, on the trainer it will depend on the resistance setting for how hard it is to maintain that speed and on the road it will depend on wind and incline.

In general, to go fast for longer distances, let's say 20+ mph for 1+ hrs, you need a cadence of at least 80 to 90 rpm. However if you are just starting or if overweight it will be difficult to move the legs that fast.

Low cadence is stressful for the muscles while high cadence stresses the cardio system.
But if you want to go fast for long periods of time, high cadence is the way to go.

For now you shouldn't be going that fast that you can only do it for 1 minute. Concentrate on a pace that you can maintain for 1 hour and at a cadence of 80-90? This should stil have you breathing harder and sweating.