If you're doing a loaded tour you need a bike with toe clearance when you mount the front panniers. Often a mtn bike is a little tight but people do tour on them. You could go the trailer route if you really wanted to stay with the 4300. I'm more comfortable on a touring bike and if you are going to a Trek shop check out their 520, Pete and I both ride them.
foot^3 knows more about bike fitness than I do but I'd try to get to the point where I'm riding 3 days a week with one serious mileage day per week. You can get fit on tour as long as you don't hurt yourself pushing too early. Its nice to know how much mileage you are comfortable doing per day. Pete and I averaged about 65 miles per day on our Irish tour. At that target you don't have to ride hard and you can have that second pint at lunch. The advertisements in the back of my touring magazine have personals for folks who want riding partners for long tours or parts of tours. I saw one in there today who wants to average 140 miles per day
Are you camping or motel / B and B?