Five years ago I bought a road bike, joined a club, and signed up for an event in 7 months with 15,000 ft of climbing over 5 mountain passes and 120 miles. When I bought the bike my fitness was built on doing 2 high-intensity spin classes a week plus another road ride or two on my mtn bike. Was 95kg and roughly 235W FTP but no real endurance, and everything out my door is pancake flat but the Sierra mountains are 45 minutes from here. Certainly not ready for my first flat century, let alone one with 10,000 feet of climbing. Posted some thoughts about it here:
Hope that helps.