Above is the Lethe River System in-game (portion of a map from KeyBlade999 on GameFAQs). Okay, it does that thing a lot of fantasy river systems do where there's a lot of splitting that isn't occurring in likely places for such to happen (e.g. low-lying coastal deltas), but we'll just. Work with that. That's fine. That's not a problem, we can just gloss over that. This is the problem:

This roughly is the path that Banon, Edgar and Terra ultimately take to escape from the Returners' Hideout northwest to Narshe. In-game they are clearly traveling downriver, with the current, and the speed of such travel is part of the point as they're trying to quickly escape. …but. By all accounts that river should be flowing largely southeast, emptying into the sea near Nikeah.

What the hell. I brought up my woes to a friend and they joked that obviously the topology down around the Returners' Hideout and north of Nikeah is WILD. ...okay. Alright. Let's do this. I'm no hydrologist, but let's roll up our sleeves and edit the map a bit and Frankenstein ourselves a solution that lets us have a downriver trip at least most of the way toward Narshe.

Approach to Magic

Being the magic-infused soldier of the team, Celes learns sixteen spells naturally by leveling up. She learns a couple basic white spells early on (●Cure and ●Poisona); the ○Blizzard / ○Blizzara / ○Blizzaga chain; and at high levels, ○Holy, ○Flare and ○Meteor. She also learns a variety of “gray” magic, including debuffs (◎Imp, ◎Confuse), buffs (◎Protect, ◎Haste, ◎Vanish, ◎Hastega) and two that are harder to categorize (◎Libra and ◎Berserk). Compared to Terra, the general’s spell selections are more tactical.

She was trained to use her magic to the best ability the Empire could offer, which presumably included a lot of experimentation; this may be part of why she learned to seal magic with her sword. It’s not really a proper inheritance of a magical tradition, but she’s got clear aptitude for and experience with it as borne out by her high bonus MP, Magic Power, Magic Defense and Magic Block stats. 

I’m sure she’d take to learning magic from magicite readily, and diversify her spell knowledge easily. This would be especially critical in the World of Ruin where, post-coma, her physical abilities would have deteriorated.

Probable Esper Pals

Arguably, Celes has a connection to all of the Espers that escaped or are “rescued” from the Magitek Research Facility. One or more of them were directly siphoned for her magical ability, and symbolically all of them were. She obviously regards Ramuh talking about the Magitek Research Facility in a way that seems personal. This, at minimum, represents a link between her and Ramuh, Kirin, Siren, Cait Sith, Ifrit, Shiva, Unicorn, Maduin, Catoblepas, Phantom, Carbuncle, Bismarck, and Seraph. Golem and Zone Seeker only appear in the Jidoor auction house after that event so they may be implied to have been sourced from the Magitek Research Facility as well. 

Other than that, let’s take a look at particularly significant Espers from her to learn from:

Ramuh is of course the Esper that made her realize what the Empire was up to, and where her power came from. It’s an awkward connection, but I think one that Celes would take seriously. A source of ○Thunder, ○Thundara and ○Poison.

Given the spells that Celes learns naturally, Shiva is quite probably the source of her magical ability, or at least a major source of such. While Celes naturally learns three of the spells she offers and Zone Seeker is a superior teacher of the remaining two (,◎Rasp and ◎Osmose), I can see her spending some time with Shiva’s magicite.

A tenuous connection, but Quetzal washes up for some reason on the shore of Solitary Island, where Celes fished for Cid and (assuming Cid’s death) encountered the bird that inspired her to keep living and search for her friends. The Doylist answer is that it’s just a reward for backtracking to a place you’ve got no narrative reason to return to; but one could easily devise a Watsonian explanation as well. Celes naturally learns ◎Haste and ◎Hastega at levels 32 and 52 respectively, but Quetzal can still teach her ◎Slow, ◎Slowga and ◎Float.