The 'even' in this sentence would be taken as 'before the event occurred/happened'. So this sentence is saying that before thinking about the 99 out of a 100, he blundered through the old world for a third of a century

I can best explain by using a simple example. "This smartphone costs less than $100" is a simple statement of fact. "This smartphone costs even less than $100" communicates two ideas at the same time: a) This smartphone costs less than $100, and b) $100 is a very low price. The smartphone is cheaper than cheap. In your example, "even" strengthens the statement by saying more than "before I realized X." It isn't just before I realized X. It's before I began to realize X.