We also don't know what rate a Time Lord's body ages at. Remember, the first's regeneration was the only one caused by old age. For all we know, a natural life for a single body of a Time Lord is 850 years.

He did spend his childhood and (one assumes) a good part of adulthood on Gallifrey not putting himself in danger and saving the universe.
Also, in the Fifth Doctor adventure 'Mawdryn Undead' (an episode I just got to! Woop woop relevant progress!) he says that he has regenerated four times and therefore only has eight regenerations left, just enough to
[SPOLIER ALERT] release the eight exiles from their imortality and allow them to die, removing his remaining regenerations in the process and leaving him Non-Time Lord. [END SPOILERS]

So The First was the first according to The Fifth.