he did have an outline concerning the prequels a long time ago. on the ep 1 dvd he shows the notebook he has the outlines written in. he says he also wrote american graffitti in it as well.
he didn't have any fleshed out stories yet. but the basic plot was in his mind. once he went to write the ep 1 rough draft in 1996 or 7 (i forget), he made the basic story arc. but certain things like anakin and obi-wan's relationship/ the big duel etc. were already thought of back in the 80's because several of the spoilers are very similiar to the original novel version, ala lava etc.