Going from 71 to 100 miles should not be a big step: It's always much easier to go further than to go faster than you're used to. It just takes longer. My key experience was a ride I joined that started early in the morning, when I realized that if you have enough hours left in the day, you can go pretty far at your own pace. As long as I eat and drink sufficiently throughout the ride, I can keep going comfortably as long as there is daylight.
I did 300 km with a longest training ride of 235 km and the distance was no problem (though lack of sleep was). Basically once you're doing 50 km or more and you have a bike that fits you (good saddle, correct height of saddle and handle bar, etc), the main things to pay attention to are regular eating and drinking and keeping a sustainable pace. My advice would be, prepare for a century next year by doing one by yourself or with a friend this summer. There is nothing magical or scary about 160 km if you follow the tips about eating and drinking.