The fog was so thick I couldn't see an inch in front of me.
Literal
Fog [subject-が] very was-thick [because-ので], 1-inch ahead [also-も] couldn't-see.
一寸先 = 'just ahead, an inch ahead,' with 一寸 being an old unit of length (~3cm). The phrase 一寸先は闇 ('an inch ahead is darkness') is a proverb about the unpredictability of the future. Here it's used literally for visual obstruction. も adds 'even.'