Roughing It is a common way of saying, "persevering through the grind of life." The title signifies the most important element of the auto-biographical story is Twain's gritty willingness to push through life's disappointments (depicted as a failed journey as a gold prospector in the Gold Rush), challenges, and eventually, rewards (Twain discovers his bliss while reporting on the undeniable beauty and cultural beauty of the Hawai'ian islands).
Twain uses his experience of disappointment to fuel his future endeavors, and it's important to note that after his initial failure in Nevada, Twain re-establishes his confidence while working a couple years in an office somewhere in Virginia, but then he tries again. This willingness to work through defeat is perhaps what Twain means in the words "roughing it." After his second failure, though, he learns from the defeats and reorients his expectations from life. Instead of wanting to get filthy rich by finding gold, he will simply use his best God-given skill to earn an honest salary.
Then he finds himself in absolute paradise. While in Virginia, Twain was not thinking about Hawai'i. He didn't plan for Hawai'i, because he thought he'd be prospecting. So then when suddenly Twain finds himself in Hawai'i, it is as if he has been rewarded for a wise decision, in an almost literal paradise. He gets to work by doing something that he loves, and his actual daily job is simply to learn more cool things about the islands. He has "struck gold" after all. At the end of the novel, Twain urges young people to get out of their homes and go adventure in life. He says for them to move to an entirely new place and establish themselves their in perfect independence.