don't reduce your cadence, select an easier gear.

actually 70-80 is a touch low, shoot for 90-100, and yes, choose a MUCH easier gear.

FWIW I also have a hard time staying in anything under zone 3, it just feels super slow and like I'm not even working. they say that it will build endurance over time, but I also don't have the time in my schedule to roll around at 12mph for 4 hours, and it's not very fun. I plan on training long hours at low perceived effort this winter on the rollers . . .