。
I walked myself into total exhaustion.
Literal
Walk-and worn-out [into-に] became.
へとへと is a 擬態語 (state mimetic) for utter exhaustion — 'worn out,' 'dead tired,' 'pooped.' Like many mimetics of physical/mental state, it combines with になる ('become') to mean 'get into the state of X.' Paired with 歩いて as the cause: 'walked to the point of へとへと.'