Ride at least every other day although 6 days a week is better.

Add 10% to your distance each week, except for the 1 in 4 weeks you have lower volume and/or intensity to recover and adapt.

Have a longer ride weekly that's twice as long as you ride the other days.

By the time you're in good shape you'll be riding 25-30 miles most days plus your 50-60 mile weekly not-short ride. If your planned ride is somewhat before then you can shift miles around between days - 0 miles on Friday and 63 miles on Saturday is about the same as 21 and 42 although a necessary stop for water is more likely. Long before then you can probably get by riding your weekly total in one day and/or going 50% farther than your longest ride to date although you won't enjoy it as much. Single rides approaching your event distance are more about discovering things which aren't issues on shorter rides than conditioning - you can get away with a lot of fit/clothing/eating/drinking mistakes on shorter ones and will enjoy your longer rides more without being surprised by those things. I rode my first century at a good pace when I'd never done more than 50 miles at a time and was riding 120 miles a week.

Don't change your bike or other equipment immediately before a longer ride.

Ride your own ride. You'll be unable to keep up with faster riders over the full distance, and will find your own appropriate uniform pace much less fatiguing than starting too hard and having to back off.

Wear shorts which are comfortable for the distance. I find excessive squishy padding less comfortable after 2-3 hours than flat synthetic leather chamois after 9+ hours.

Bring extra clothes if the weather will change while you're riding due to clouds, time of day, or elevation. I forgot about temperature lapse on my first century in 19 years, left my gloves home because it was a pleasant 53 degrees when I started, and had to warm my hands in my arm pits because the temperature dropped to 43 after I gained 2600 feet.

Drink when you're thirsty. Weigh yourself before and after when riding to see how that works - you want to end rides at the same weight you started them. Dehydration is bad for your performance and health. Too much water isn't as bad but you'll enjoy yourself more when you don't have to stop and pee too often.

Bring two 25oz water bottles. It's more fun when you don't need to stop to refill too often. For 100km you don't need a sports drink. For longer rides plain water can still be fine - notice that a lot of energy bars have all the electrolytes you need.

Eat if you need to. With enough training you won't need to on a 100km ride. Without you may need food. A little at a time is better. A mix of high and low glycemic carbs works well. Regardless of how much energy you're using you can't digest more than about 250 Calories an hour so once rides are long enough to require food you're better off starting after the first hour.

Don't eat a large breakfast. Too much food and/or things which aren't easily digested can give you digestive trouble.

Be careful, the metric century is a gateway drug to 100 miles, 200 km, 300 km, 200 miles, and beyond.

Also note that you don't need to wait for an organized ride to do any of those distances. The internet makes it easy to plan routes from your desk with minimal surprises - ridewithgps.com lets you use the opencyclemap maps which show official bike routes and things of interest to cyclists like water sources. You can see satellite pictures and inspect road surfaces/shoulder widths via Google street view there or at Google. You can google for 24-hour grocery and convenience stores for re-fueling. It that's too much work you can search for permanents at rusa.org and on local randonneuring groups websites', usually ending up with .gpx files plus cue sheets for navigation on routes experienced cyclists consider good. Previous years' organized ride routes are also available.