Infinitives were used instead of the correct verb forms. Infinitives are using the preposition 'to' + a verb. In the second part of the sentence, is says, "to eat in the office and to work alone". This does not work in conjunction with the first part of the sentence. They need to mirror "staying late", so the sentence should read, "Long work days mean not only staying late but also eating in the office and working alone."
