Singhs during Bhai Sahib jee's times often used to sit through Siri Akhand Paath Sahib, without getting up or eating anything (except Degh during start, Madh and Samaptee). Their surthee used to stay in high spiritual realms. Bhai Sahib Randhir Singh jee listened to many Siri Akhand Paath Sahibs during his lifetime, without sleeping, or even changing the sitting posture. After the completion of Siri Akhand Paath Sahib, there used to be Siri Asa kee Vaar Kirtan. After conclusion of Samagam, Bhai Sahib used to first do Gurmat Ishnaan and then do all his due Nitnems. He even used to listen/do missed Siri Sukhmani Sahibs and only then he would eat anything or rest.
Taking cue from Bhai Sahib, if one sits through whole of Siri Akhand Paath Sahib, it is fine to catch up with Nitnem Paaths and Gurmat Ishnaan after the conclusion of the Samagam.
Bhai Sahib Randhir Singh jee has written in Jail Chithiaan that when he listened to first entire Siri Akhand Paath Sahib in one sitting, he wished for Darshan of Siri Guru Gobind Singh jee and that's what he got. So great is the reward of listening to Siri Akhand Paath Sahib. But listening to full Paath does not mean just sitting there. Bhai Sahib writes that by the Kirpa of Guru Sahib, he listened to every word, every Pankiti for 2 days, with full concentration. That's why he got Darshan of Siri Guru Gobind Singh jee.
Bhai Sahib has written that in olden days prior to taking Amrit, Abhilaakhis were asked to try to listen to whole of Siri Akhand Paath Sahib. Fortunate ones who used to obey this Hukam used to get blessed with many spiritual experiences at the time of Amrit Sinchaar.
Having said that, it is not possible to sit through Siri Akhand Paath Sahib with will-power but it is pure Kirpa that enable one to listen to whole Paath. One should beg for mercy and Kirpa from Vaheguru. Then alone one can do such Sewa.