The rule of thumb is that if you can ride 70% of your target distance about 40-45 miles, you should be ok for a metric (63 miles) assuming the hill profile is similar. If you spin the way you ride, then spinning is a good proxy for cycling. You can cheat at spinning though by that I mean riding along with little tension on the fly wheel. You can not cheat in cycling because riding a mile is riding a mile and there is no way around it. Nothing is going to prepare you like actually logging the miles on the bike.

Any kind of aerobic exercise that uses your legs that you do during the week will help: running, jogging, walking, stairclimbers, spinning and so on. But try to get out every week end and ride for at least 20 miles and hopefully on both Sat and Sun. The more riding the better.

You want to always carry at least one water bottle on your bike and 2 is better. You need to drink often. Try to drink a bottle per hour. Now I can ride a metric without eating, but that does not work for most people. Most rides have SAG stops every 15-20 miles. Stop at the SAG stops and drink some water, eat a snack, relieve yourself and get going. Try to make the stops relatively short. If you sit too long, it is harder to get the muscles going again. I don't eat much of anything before most rides. For centuries, I will eat a banana and maybe half a small bagel before starting. Everyone is different. That is why it is good to get some rides in to get an idea of what you need to do. Also, it gives you an idea of what sort of foods work well for you. I know some people who can not stomach any solid food and get calories from GU or something similar. Me, I can eat darn near anything on a ride.