I agree with most of the things laid out above me, though I'd argue the 'one panel story' idea is more of a general sequential art exercise than a good 'first project'.
Don't get me wrong, I think it's a good exercise and you should absolutely do it, but for me when I get an idea for a big epic long-form narrative, I have a powerful itch to do things relating to that narrative or else I just get annoyed/impatient/distracted with other projects.
in the particular case of epic fantasy because whooo boy do I have the same problem, I'd recommend picking a small element of the story at large and focusing on that for something self-contained, but in the same universe.
Like, say, is there any particular location the characters visit in your story that's interesting/appealing to you? What sort of fun side characters show up there? Maybe a 5-page slice-of-life story about that maid in the background of the palace trying to deal with a particularly stubborn stain on the princess's dress.
Maybe one of the regular footsoldiers in the army has a particularly rough day of training and has a conversation with his girlfriend about whether or not he's cut out for this job.
Maybe a hunter-in-training tracks down a rare creature on the outskirts of a city.
You're the one making this story, so you'll have to determine what parts of it are interesting enough to warrant a side story and what that story is about, but this lets you explore, flesh out, and work within the fantasy universe you've created without jumping into the deep end.
This way you aren't deprived of the really cool concept you're so excited about, but allows you to get in and out and have a finished product that you aren't in as much risk of getting bored of/burnt out on. Speaking from personal experience here: Epic fantasies are awesome, but they are a REALLY big commitment.
Problem is, if you don't start on it, it becomes a brainworm that will eat away at your concentration and make it hard to get invested in other projects because 'oh man I really wanna do that scene with X' or 'man, this fight against character Y is gonna be so cool' etc. etc.
Doing stuff that's in the same universe, but doesn't require that level of commitment may let you have your cake and eat it too, in a way.