LAST EDITED ON Jun-08-10 AT 07:54 PM (EDT)
 
Well, they do have untelevised adventures, and if the Professor's age is described the way the Doctor's is, long periods of time can pass off-screen (the Second Doctor tells his companions he's about 450, whereas the Fourth Doctor is around 746 when he meets Romana, and the Seventh Doctor is 953 in Time and the Rani), so its not unreasonable that she could have been separated from her companion(s) for a few centuries.

“They planned their campaigns just as you might make a splendid piece of harness. It looks very well; and answers very well; until it gets broken; and then you are done for. Now I made my campaigns of ropes. If anything went wrong, I tied a knot; and went on.”
-- Arthur Wellesley, First Duke of Wellington