I've been snooping around the forums for a few days and I've decided that I need to just ask the question I've got, and hope I get an answer.
I'm sixteen, and I've signed up for the "America by Bicycle" ride, from St. Joseph, MO to Portsmouth, NH ride. I'm in great shape - I swim, I run, and I bike a little - but I need to do some serious training for this as each day is going to be 80-100 miles. I just bought a bike - a new mercier corvus

Frame Reynolds 520 double-butted High Grade Steel with eyelets & rack braze-ons
Fork Reynolds Ouzo Comp Carbon Fiber / Aluminum Steerer
Headset FSA Threadless for Cane Creek
Crankset Truvativ IsoFlow triple 52/42/30T
Bottom Bracket Truvativ sealed
Pedals Alloy Road with Toe Clips and Straps
Front Derailleur Shimano 4400 for Triple
Rear Derailleur Shimano NEW 105 for Triple
Shifters Shimano 4400 for 27-speed
Cassette/Freewheel Shimano 9-speed 12-25T
Chain Z-9000
Hubs Formula Aluminum Black, sealed bearings
Spokes Stainless Steel
Rims ALEX DA-22 Aluminum double-wall black anodized with machined braking surface
Tires Kenda Racing 700x23c
Brakes Aluminum Dual Pivot
Brake Levers Shimano 4400 STI
Handlebar Ergo Butted Aluminum 6061
Stem Aluminum Threadless
Tape/Grip Cork
Saddle Velo Racing
Seat Post Aluminum Micro-Adjust

And I was told by my grandfather (who did this ride a few years ago, and who is going with me) that I needed a 12-30 rear cassette. I asked the woman who built the bike and who was selling it to me if the dérailleur could handle the 30 tooth cog, and she said it could, but that I might need to change the crank. I'm not sure what I would need to do/buy to put a 12-30 cassette on my bike. I am very good with my hands - I build battlebots, and I have a lot of experience with building things and working with metal - so I could probably make the necessary changes myself (if there ever are any).

Can someone please help me out? This is my first road bike, so I am not familiar with all the terminology and technical jargon that a lot of people use when giving advice, so simple language would be appreciated. :]

Thanks so much