。
I'm tired from walking.
Literal
Walk-tired is.
歩き疲れる is a compound verb from 歩く + 疲れる ('walk' + 'get tired'), meaning 'to get tired from walking.' This is the productive [V-stem]+疲れる pattern for 'X oneself out doing X': 遊び疲れる ('get tired from playing'), 待ち疲れる ('get tired of waiting'). The ~ている wraps it in the resulting state.