Endurance?

This past weekend I had a 3 hour training ride (Spinning) for my upcoming Spin Odyssey event (http://www.spinodyssey.org/).  I'll admit, I was NERVOUS.  While I have taught 3 classes in one day, I have never done three hours consecutively and between you and me, I wasn't even sure I could do it.
But, I did do it and it felt great!
The difference between a 3 hour ride and a 45 minute ride is also the subject of this post, ENDURANCE.  Like many spin instructors (and students), when we teach (or take) a 45 minute class, the goal is to work hard and sweat.  I feel like if I don't make the class work hard, they're going to leave feeling cheated and then maybe never come back to my class.
However when you're riding for 180 minutes, the goal has to be different.  The first hour of the ride was completely "endurance" training.  While we moved through different positions and resistance, the goal was to never get our heart rate over 75%, which for me is somewhere around 145 beats per minute.  Sometimes this meant going slower than my natural inclination or using less resistance than I'd normally strive for.
So while it was very different than what I'm used to, I enjoyed it.  It forced me to be in touch with my body and how it was feeling.  I was one of the few people wearing a heart rate monitor so I was able to compare how I felt with my actual effort.
The next two hours were a bit more varied, but still with more of an endurance theme than an all out effort.  Which when you think about it, is really how a 3 hour ride has to be.
I spoke with my sister after the ride.  She's currently training for a half marathon and then perhaps the full.  She's on a pretty structured schedule and told me that 90% of our training should in fact be endurance, especially when the goal is a significant distance or time.
It makes perfect sense.
However my dilemma still comes back to my 45 minute class.  Most of the students don't wear heart rate monitors so i'm not sure that they'll be true to themselves in how their body feels and then end up leaving the class thinking it was "easy".  The mentality of a casual spinner is to work hard, so I'm just not sure that the level of student I teach will be accepting of the fact that endurance is a key part of any training regiment.  But really, that's my issue.
I definitely plan on incorporating more endurance training in to my rides, it's just a question for me, of finding the right balance.