You understood, that I promote my Medics to Medic 3 first, then to Championship for lvling, and that I also use multiple ones of these, right. Getting the 2 promos can be achieve by having a 7 XP unit, after that, I let the unit fight 98% battles as those give 2+2 XP with Championship, so Withdrawl and extra-movement usually follow shortly after that, reaching 50-70 XP is not unusal for Medics in my games.
Might be a Huge Marathon thing though, more opponents, more units to kill equal endless XP and GGs.
I do think about completely skipping Supermedics on Marathon though and settling all of them, as the healing of a Medic 2 is not that bad, at least not on that speed. Most units take about 3 turns to heal to full with Medic 2, the Supermedic only saves 1 turn in about 50% of the cases, so not that much as the word "Super"medic would suggest.
With fast warfare, I found that it's more important to have decent numbers of healers, like 4-5, and set up main-healing-centers aswell as secondary healing centers. Saving 1 move because one has 1 more medic in place is better then having to move units to the Supermedic, I only move the very heavily damaged units to that one.
And having a fighting Supermedic makes sense, as he has to stay and heal in a city, taking the Medic completely out of fight is like loosing a really good unit imo.