(disclaimer haven't slept for a while)
I wouldn't even call the "normal" run crushingly difficult. I'm by no means good at rhythm games, but I beat the normal run in about two weeks (interrupted by family travel) of play. It's difficult at first, except it gives you easy enemies at first. And each stage is reasonably short, like 10 minutes I think (if you succeed, otherwise it's shorter). That goes for the final stage, though of course there's a boss.
And then, without spoiling anything, there's another two story-centric runs (think the negative castle in Symphony of the Night, but with another twist).
Full disclosure: The 3rd story run is pretty absurd. I haven't beaten it. But it was no herculean feat for me to beat the 2nd run... Just a matter of patience and learning a new mechanic.
There are also several other characters who you can run as, who aren't related to the story (or are they). They're generally a little bit easier, except for monk, fuck monk and gdammit I'll never unlock Bolt will I?
To summarize this game: Once you know the pattern, it's still tough but not too tough for a scrub like me to reach a very satisfying victory... or two.
So basically what BurnedToast said - it's ALWAYS fair difficulty. You only get exposed to new enemies 4 times, and then you're allowed to practice against them indefinitely.