Usually when a ride is converted for indoor use, it gets shortened to 1 hour and the intensity goes UP a bit.  This is because when we first built the option for indoor conversion, most people were not on smart trainers and indoor training had not taken off the way it has now.  So, in future updates to VeloPro, we will simply allow for the full-length conversion of the actual workout.  

For really long rides (4+ hours) we plan to develop functionality that gives riders a choice of converting the ride as it is, or reducing it by a given amount and having VP adjust the rest of the schedule to suit.