Try to do a twenty miler this week and a thirty or at least a 25 before the event. Two numbers that can have motivational meaning in that range are a marathon and 50 km.
If you take it slowly and have support you can probably get away with skipping ramp up.
But to illustrate what can happen, I've done 100 mile rides in past years and after a slack winter was doing 50s while there was still snow beside the roads, but then slacked off again. Decided it was time to get going and did a 20 mile loop without issue.
Then I went for a fifty after having basically ridden only twenty miles in an entire month.
Ouch. I got it done, but started to have cramping issues on the second climb. The third saw me sitting in the curb near screaming and trying to find some way to stretch out locked up muscles until I could even walk to where I could bail out downhill to a flatter alternative route. It took a few rounds of electrolyte tablets and a lot of pacing and strategy to finish that ride.
Your body can extend to a lot, especially if you're careful of how you care for it - preferably using things you've tried before.
You're also getting into the territory where cycle shorts and chamois cream make sense - even more so for a body that isn't used to it than for one that is.
And don't forget fit issues.
Or sun