The longest ride I did prior to my first century was about 70 as well and that century was the longest ride I'd done before doing RAIN, which is 160 miles. I just did a 95 mile ride last Sunday and my longest ride for this year prior to that was 48 miles. I've never really followed a training plan per se. Like Makbike, I've generally just gradually increased my mileage and taken rest days as needed. At this point, I'd worry more about the terrain than the actual mileage.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher