Just a piece of advice: don't wait too long, I've managed to run out of fuel on a sunday early afternoon. No fun in rural areas, believe you me
Generally I agree to what backofbeyond and Flyingdoctor have said - you'll be fine on motorways and don't count on machines taking anything other than French credit or debit cards. I've tried German and US and neither have worked. On the other hand if you'll carry enough cash with you it shouldn't be too hard to find a local with a card that works.
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"