Riding a metric century is extremely doable for most new cyclists given proper training, preparation and equipment. It’s a short enough distance in the grand scheme of things that you can even get away with riding a daily commuter bike (as opposed to some form of touring or road bike) as long as it’s one you find comfortable in every day use. If your bike is even slightly uncomfortable in day to day use, 100 km and multiple hours of ride time can turn into a painful experience so make sure your bike’s fit is well dialed in.
Just hopping on your bike one day after weeks of not riding and going 100 km is probably possible, but definitely not recommended from a health and enjoyment perspective. When I rode my first metric century last September I had been averaging roughly 20 km a day and over 100 km a week for a couple months leading up to it. As my fitness improved each week I would gradually incorporated longer rides and rode a couple 50 – 60 km rides by late August/early September. That ended up being more than enough training preparation for me and I felt strong throughout the entire ride
One key difference in a multi hour ride like a metric century from daily riding is that you also have to factor in water and nutrition. If you’re not going to have access to water to refill along the way you’re going to need to bring more with you. The same goes for food. While you can get away without worrying about food too much for shorter rides, I find that I start to get pretty hungry after a couple hours of riding and 100 km is going to take most new riders at least 4 hours. Packing things like energy bars, bananas and trail mix and then making sure to partake every hour or so will make sure that you have a solid energy supply the whole way through.
The final piece of the puzzle when riding 100 km is making sure your bike is in good working order. When attempting a metric century, make sure that everything on your bike is tuned up and your chain is well oiled. A spare tube, pump, a couple tire levers and the knowledge of how to use them all to fix a flat tire is also important and will allow you to get to the end of your ride as planned.