I have been in the process of doing the marathon slowly but surely over the past year and a half.
I play XIV when I can and I enjoy it, but hardly have time for one MMO, much less a second one. I decided that trying to do XI was too big an ask, so I opted to replace it with Tactics (the PSP War of the Lions version specifically).
Tactics is a much-praised game that, despite not being numbered, is considered part of Final Fantasy's core identity (and comes up a lot in XIV), and it provides a nice tie-in to 12, both being Ivalice games, so I thought it worked well.
The only difficult part is that there is no PC port of Tactics. I had to emulate it. Though I do know that there are iOS/Android ports of it which may work better for some.