Preceding Word Length

I have to make a function that takes a single argument word and returns the average length(in characters) of the word that precedes word in the text. If word happens to be the first word occurring in the text, the length of the preceding word for that occurrence should be zero. For example

>>> average_length("the")
4.4
>>> average_length('whale')
False
average_length('ship.')
3.0 

This is what I have written so far,

def average_length(word):
    text = "Call me Ishmael. Some years ago - never mind how long..........."
    words = text.split()
    wordCount = len(words)

    Sum = 0
    for word in words:
        ch = len(word)
        Sum = Sum + ch
    avg = Sum/wordCount
    return avg

I know this isn't right at all but I'm having trouble of how to approach this correctly. This question is asking me to find every instance of word in the text, and when you do, calculate the length of the word immediately before it in the text. Not every word from beginning to that word, just one.

I should have also mentioned that all the tests will only test my code using the first paragraph from 'Moby Dick':

"Call me Ishmael. Some years ago - never mind how long precisely - having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world. It is a way I have of driving off the spleen and regulating the circulation. Whenever I find myself growing grim about the mouth; whenever it is a damp, drizzly November in my soul; whenever I find myself involuntarily pausing before coffin warehouses, and bringing up the rear of every funeral I meet; and especially whenever my hypos get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off - then, I account it high time to get to sea as soon as I can. This is my substitute for pistol and ball. With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship. There is nothing surprising in this. If they but knew it, almost all men in their degree, some time or other, cherish very nearly the same feelings towards the ocean with me."