i'll weigh in here. i'm not as concerned with the 20 miles or the speed or the amount of time in the saddle. it's what you are doing during those 20 miles. a steady pace on a stadionary bike is a poor (IMHO) training method for any outdoor biking, be it mntn or road.
i feel that you should have as an example:
tuesday: steady pace
wednesday: interval
thursday: speedplay
friday: light spin/recovery
saturday &/or sunday: endurance, 90 minutes or longer
monday: off
you need to shock your body into making gains. the status quo will do you no good once you factor in bike handling, wind, terrain, etc.
train like your race you want to ride. does it start steady then turn uphill then gets technical? simulate your race or training.
you sound like you have a good solid fountation. i'd be cautious about riding too much indoors (not good for the mental aspect) --but every workout should have a specific goal. don't just get on the bike and pedal. have a specific plan or each ride.
and with those HR numbers, you were given both a gift and a curse that you have to craft to your advantage.