(1) There are usually three "correct" ways to punctuate that sentence:
(a) She was petite and fit and sensible.
(i) People usually don't speak or write like this often.
(ii) It's fine occasionally. It certainly gets one's attention, doesn't it!
(b) She was petite, fit, and sensible.
(c) She was petite, fit and sensible.
(i) Nowadays, probably most Americans prefer (c). They ask: why use an unnecessary comma?
(ii) But everyone agrees that you need to use a comma if it is necessary to avoid confusion.
(a) One book gives this example: There were blue, green, and red flags.
If you did not put a comma after "green," then that would mean one flag was a combination of two colors:
There were blue, green and red flags. That is, one blue flag; one green and red flag.
So if you are speaking about three different flags with its own color, then it would be better to use the second comma.
Have a nice day!