What have Devāpi and Maru been doing in their over 5000 year long lives? Also, how did Devāpi get to have such a long lifespan?
Since they are both described as being the originator of the Kṣatriya in the next Satya Yuga, it seems to imply they have children together, either by the gender-swapping in the Kalki Purāṇa, or while both staying male like some of Mitra and Varuṇa's children. I think it's while they are both male, as that nicely fits fit the major theme throughout the future prophecies of revealing The First Human as Sarvavarṇyantaka and eventually Sāvarṇi, as it would end the Sūryavaṃśa and Candravaṃśa.
These only show their story before they were even longer-lived than their siblings and after the events of the Kalki Purāṇa begin, which leaves a massive stretch of time where they are unaccounted for.