What does this quote mean?

"iron rust from disuse, stagnant water loses its purity, and in cold weather becomes frozen; even so does inaction sap the vigors for the mind.""iron rust from disuse, stagnant water loses its purity, and in cold weather becomes frozen; even so does inaction sap the vigors for the mind."

Read the passage. when i consider how my light is spent ere half my days, in this dark world and wide, and that one talent which is death to hide, lodged with me useless, though my soul more bent to serve therewith my maker, and present my true account, lest he returning chide; “doth god exact day labor, light denied? ” in line 7 of sonnet xix by john milton, the speaker asks, “doth god exact day labor, light denied? ” what does his question mean? why must god make us suffer to work? what shall one charge god to work in the dark? how does god expect him to work when he is blind? is god trying me?