I'd disagree about needing a higher cadence for a longer ride, my cadence drops the longer I ride for as the intensity also drops. My longest ride to date my cadence was around 70, whereas yesterday for the GCN race it was over 100 and commuting it sits around 85-90.
That's the reason to have a high cadence for longer rides. I start out at 95rpm but by the end of the day I can let cadence taper and still be able to keep going. If you start at 70 rpm, there is good chance your legs could be knackered. If you've got experience and already built stamina then if it feels comfortable start at a lower cadence.