Marcus reminds himself of his duties. The sense I get from this passage that a person should develop this inner sense to desire to fulfill all duties and obligations willingly. All action should be of your own free will. All action should be unselfish. All action should be critically analyzed and all action should be carried out with integrity.
Furthermore, Marcus reminds himself that not only should his actions meet the above standards, but his thoughts should not be dressed up and he should not be meddling.
He speaks of his inner divinity or daimon as his guiding light.
He should be cheerful and self-sufficient.
Lastly, he is to act in duty and not wait to be acted upon or commanded to carry out his duties.
(See also Citadel pp. 123, 265, 289)
(link to some fancy wallpapers with the "stand straight" quote)