Miles and speed don't mean a thing on an indoor trainer. It's watts (power) and time spent on the machine. Heart rate is certainly an indicator of how hard you're working, if no power info is available. I've always found indoor training so boring that tolerating a full hour was about all I could stand at one time. Training for 4-5 hours a week in the winter only helped to limit my fitness losses, compared to riding 9-15 hours a week in good weather. I didn't "gain" anything from it.