If my bike is a little small, will it make a long bike trip harder? I can't afford a new one!?

I bought a Trek 4300 bicycle about 1 year ago, thinking I cant grow that much more. I was in fact wrong though, as I grew to 6'0. I am now 6'0 riding an 18" trek 4300. Will this make my trip uncomfortable?(about 400 miles), harder, or less enjoyable? Also, what size should I have?