You didn't mention your weight, which is a factor on wheel suitablity.

FWIW-- I did several long tours in Oregon and Washington (not flat) when I was young on my 1973 Puegeot PX10 competition bike. It was all I had at the time. The gearing was 52/40 front and 14-28 rear 5 speed freewheel--Fully loaded and in some pretty hilly terrain. It did just fine. However, I did build a pair of clincher wheels, because I was concerned about the fragility of tubular wheels and tires, not to mention the expense. The 14-28 replaced the 14-24 or 14-21 I usually ran on the bike.

The point is that the advice of some of the other poosters to "just go for it" is proably pretty good advice.