There is also a direct bus line Toulon - Aix with a limited number of departures during summer holiday time. varlib.fr/cg83/…FH19.pdf
From Toulon to Aix the drive is about one hour 15 minutes (some 55 miles only, though). Add the time you need for receiving the car. There are 3 or 4 toll stations on the way. At the toll on A52 towards north, you pick a card from the machine, which you then enter into the slot at the gate at the toll station of A8, some southeast of Aix. Then you pay the indicated sum to open the gate. I use my VISA chip card at the toll gates, but coins and notes are ok, too. Choose a toll gate accepting cash, in case your card does not work there. The several outer gates (leftmost lanes) at the A8 toll station are for "carte bancaire" only, no coins or notes accepted. The toll stations may be very crowded on occasions and constant changing of lanes etc. make things worse there. Do not pick "T" or "telepeage" lanes, as these is for cars with automatic transponders only.
Access Aix from the Sortie 30 (AIX-CENTRE AIX-PONT DE L'ARC) and drive towards "Centre ville" north/northeast. There is an entrance to an underground parking garage on Avenue des Belges ( a green sign for the underground parking facility some 500 meters before the great roundabout at Pl General de Gaulle, at the west end of Cours Mirabeau. The "pit lane" immediately descends to the underground parking. Do not hesitate to drive to the deeper levels, as it will be easier to find free space nearby the correct elevator. The arrangement in the underground garage is not the most self explanatory I've seen, but manageable (at least I'm back home from there). As you leave the car and exit the garage by the elevator, choose the one taking you to Place du General de Gaulle / Cours Mirabeau. This roundabout and the fountains there are the core of the centre of Aix, and the tourist office is there, too. The another entrance and exit to this underground garage called "Parking Rotonde" are at Avenue Joseph Villevieille a bit aside, some SW from the roundabout.
Aix has a very good website http://www.aixenprovencetourism.com There you will find loads of information also in English.
At Place du General de Gaulle there is a "petit train" taking you around the major spots and there is an interesting service by electric taxi/buses http://www.la-diabline.fr/
Regarding walking, the old town part of Aix is compact, lots of squares, street markets, shops etc. Quite enjoyable for a day trip. The tourist office web site explains all this very well and there are maps, as well. You can pick a paper map as soon as you've surfaced from the parking garage. Nice places to eat and drink, nice platanes trees etc. A very popular place for a reason.