Marseille to aix de Provence

Train. The trip can take less then 15 minutes. That's on the TGV, and you'll be delivered to the TGV station outside Aix-en-Provence. Or you can take a regular train that stops at the station right outside the historic area of Aix. That will take nearly an hour, best case.

Bus MRS Airport to Aix CenterAre you arriving at Marseille airport, Marseille train station or Aix TGV station? Your post doesn’t specify. If airport, there is a 30 minute bus to Aix Center (link above)

Barbara's idea is better than mine if you're flying into Marseille and want to go to Aix immediately--assuming the bus schedule works well for you.