64 yr old guy here. I ride my Waterford SS occasionally. Back in August I rode about the same speed as you. I averaged 17.6 mph over 14.33 miles with -2% to 2% grades.

I’m currently set up with a 49x20 and averaged 91 rpm on that ride. This 65” gearing allows me to ride 4-5% grades. I had a 49x18 on the bike but the 72” gearing was a bit tough on the 4%+ inclines.