Unless he means accumulated experience of himself and whoever taught/tutored him in whatever it was?

One of the first examples of him lying about his age

Not as bad as Borusa, though, getting through three regenerations in less time than it took the Doctor to get through one...

Yes, and every time he regenerated he seemed to change into another old man... Whilst the Doctor can bemoan the fact that he's still not ginger, I bet Borusa was cursing after each regeneration 'still got arthritis...'

Returning to the actual theme of the thread (what the Doctor did before he stole the TARDIS), the Doctor tells Cameca in The Aztecs that he is "a scientist, an engineer, a builder of things". If I remember correctly, he also says in the first Dalek story that he is/was "a pioneer" among his own people.

This suggests a more interesting past than some of the earlier posters on this thread have allowed for.