In reply to ogreville:
You should be fine, I did it several years ago with a few friends without anf much prep... First leg to Fort Augustus even included a couple of beers in the barge pub at laggan locks. One of the sections of track was quite root-ridden, rest was fine with nice gentle stretches of canal either side. Fairly full day but we weren't pushing it.
Next day was a bit harder though mostly due to the few shandies had the night before. The full Scottish before the hill climb out the foyers side of loch Ness wasn't performance enhancing (we opted for this rather than the normal route on the west side). The descent down into Inverness as a result was cracking...