Cycling - my first century ride

I have been planning somewhat unofficially to ride in the Miracle Hill Cycling Challenge on Saturday. However, it appears that we are going to be out at Lake Keowee with my wife's family. Fortunately I had not signed up for the race yet. (BTW, they expect each rider to raise $950.00 in sponsorship - is that even possible?) I stumbled onto another race in Walhalla the same day called Issaqueena's Last Ride. This is only about 10 miles away from my in-laws lake house: perfect. I have never done a race of this type of distance, and this one boasts of over 8500ft of climb over the 100 mile course. I don't know what I'm getting into, but it should be interesting! My longest ride to date is 65 miles from Lake Robinson to Saluda... I might be in for a lot of pain. I hope they have SAG vehicles!