I'll put it this way, it always comes off like a grandfather exaggerating about walking ten miles to school every day and walking uphill both ways.
I have no doubt some ideas/characters he had written early on where used for the prequels, but the vast majority of the material he created for the PT screenplays came from doing it as he went along, and trying to fit things together between the two trilogies, I know some old cenceptual McQuarrie artwork was used in ROTS (Vader on Mustafar). but IMO he no more had the prequel backstory already planned out during the OT than most of us did.
Honestly, I wish the prequels would have been made a few years after ROTJ, I think CG dummed down the prequels to the point where the actors and depth of the character driven story almost seemed like it wasn't as important as before, Yoda and the Emperor were far more interesting when they wern't doing flips and wielding lightsabers, i'll take a puppet Yoda or Jabba to a CG version, and a Stormtrooper over a Battle Droid any day...It seems Lucas was far more inventive when he was more limited effects wise.