I only ride 10 miles at a time on my trainer. Can not stand to go any longer, even in front of the TV. But I keep track by miles not time or heart rate. Hell I don't even wear my HRM anymore. After using it for a while I now know what my HR is by the way I feel, how hard I'm riding, how fast I'm going, etc. Of course a lot of it is based on the wind, hills, etc when I ride outside. On the trainer it is not that much more difficult to figure it out.

One thing about trainer or roller riding vs. riding on the open road is if you stop pedalling you stop moving period. On the open road you can stop pedalling but continue rolling forward. So IMO you MIGHT get more of a physical workout & benefit on a trainer then you might on the open road.