The journey back to the capital was not done as leisurely as when they went to Southberry.
For one, Mars and Emmelyn already got their objective, to get married and take Emmelyn to sightsee, which they did. And the second reason was, they had to immediately find Killian.
Now that Mars knew Emmelyn's brother was still alive, he needed to go home quickly and find him. It was very important to make sure Killian didn't do anything that would incite the king.
Imagine if Killian was trying to attack the king or even letting his identity known, there would be an uproar in the capital.
King Jared would feel that his family was threatened. He might investigate deeper about Killian and he would find out about Emmelyn's real identity.
Mars couldn't let anything bad happen while he was away. That's why he was in a rush to go back.
