Hey, I'm in a riding club here in Maine, called TROT...we do a lot of rides that could be possible conditioning...and fun because it's with a group! If you'd like a schedule just PM me...we do generally around 10 miles per ride, the difficulty varies, but it's usually not bad...so a good way to begin conditioning. Every now and then someone will do a long ride (more like 20 miles). As far as general conditioning goes, just get your miles in, do lots of hills(it's actually harder going uphill if you make your horse walk). I would learn some survival skills and plan carefully...definitely do your research!
One of the women I ride with is planning a trip from Maine to Cali...I'm not sure what year she's planning it for...hers may be 2013. One gadget she read about that I'd recommend to you is a tracking device you can wear so that people at home can find where you are at all times....good precaution if for some reason you needed to be rescued! Are you planning this alone, have you thought about taking a pack mule/horse?