I'll also say that 26" wheels *should* be OK (assuming the bike is in good working order and fits you) so long as you have the right tyres (I run 1.25" Specialized Fat Boys and 1.75" Schwalbe Marathons -interestingly enough, I much prefer the wider Marathons, they seem to have less rolling resistance).
I think the general opinion is that 700c are faster, however, how much faster is open to question. If you are going with pals who are super fit and you struggle already, then maybe 700c is the way to go to just gain a little bit more. if you're touring for fun and everyone is going to stay together, again, 26" wheels should be fine. However, it doesn't take much (e.g. poor choice of tyre on a 700c bike, slightly unfitter rider) to equalize things out. All things equal, I'm not convinced wheel size makes that much difference to speed.
One other point: a frame can be noodly causing efficiency issues, so if your bike is noodly, I'd recommend a change. A noodly bike can occur regardless of the wheel size -more a function of bad design or materials or a heavier rider. Another thing: if you intend to carry pannier bags, it could be your current bike will give you heel strike, though to be honest most 26" wheel bike designs don't seem to have this problem. Theoretically, since a 700c wheel is larger than a 26" wheel, this should result in longer chainstays and hence less likelihood of heel strike. In practice however, I've never had a problem with heelstrike on any of my 26" wheel bikes.
FWIW, my tourer and commuter both have 26" wheels. My preference for 26" wheels goes back to the fact that i really dislike toe overlap -because of my height, some 700c frames give me overlap. While some people seem to have no problem with toe overlap, I do -again, it's what works for you.
Anyway, back to the original question, a 26" wheeled bike should be fine assuming it is capable in all other respects for touring.
Jags
With respect it really depends on the ground you are covering, the tyres you are using and the load you are carrying.
As Hubbike said:
"The advantages of big wheels are that they roll more easily and the bikes are generally faster (all else being equal). On the other hand, smaller 26 inch wheels are stronger and so are the weapon of choice for mountain bikes, and bikes based on mtbs. Expedition tourers will often use 26 inch rims because it means that gravel tracks in far flung places are a possibility."
26" wheels are more than capable.
:-)