# from the time I was appointed

Here "I" refers to Nehemiah.

# from the twentieth year until the thirty-second year

"from the 20th year until the 32nd year" 

# of Artaxerxes the king

"that Artaxerxes was king"

# twelve years

"during those 12 years". Nehemiah is emphasizing that he did this continually for the full time he was governor. 

# the food provided for the governor

"ate the food that the people provided for the governor"