If you have to take it easy for several days after a 2 hour B ride, you would be better off doing the C ride to enable training/riding in the days afterwards instead of resting. Consistency is critical. I'd improve better having 5-6 days in Zone 2 than 1 day at threshold and two other rides during a week.
If I were you, I would try the B ride start to finish and if you want ride the next day, then no problem. If it takes 2-3 days of recovery before you really want to ride and feel solid, you might not be ready for it.
HR in the 170's for 2-3 hours is high for anyone except young and unfit. As you gain fitness, you will make more power at a lower HR due to increased stroke volume/more plasma volume, and better hemodynamics. The best way to achieve is subject to much debate.