Garrick Ohlsson managed to make a challenging opening sound gentle and suave, to the point of being downright inviting. (Pier Andrea Morolli)

When you talk about a piano concerto with such words as “longest” and “most difficult,” people tend to be put off. This is all the more true if the piece isn’t well-known, and even more true if it was written in the 20th century.

Yet some of the most beloved concertos in the repertoire are long and difficult (Brahms 2, I’m looking at you). We like watching soloists perform amazing feats of athleticism with their fingers. (“Flight of the Bumblebee,” anyone?) We like tests of endurance.

So in a way, it’s surprising that Busoni’s piano concert, which the National Symphony Orchestra, the conductor Rossen Milanov and the soloist Garrick Ohlsson played Thursday night, is not better known. Because although it’s 70 minutes long, it’s eminently diverting. And although it’s hard, it’s hard in the old-fashioned sense of being finger-busting and death-defying — and easy on the ear.

For a piece billed as towering, the Busoni concerto is actually pretty chatty. Some of its pacing calls to mind a Bruckner symphony, slathering on layer upon layer of sound, never making a statement once when five times will do. That starts with the piano’s first entrance, a few minutes into the piece, which goes arpeggiating up and down the keyboard, over and over and over. Ohlsson, rather than plunging in and challenging the heavens, managed to make this challenging opening sound gentle and suave, to the point of being downright inviting, drawing you in rather than demanding your ear.

Although some places in the score could be more concise, Thursday’s performance didn’t feel like it went on too long. That was partly due to Milanov’s control. He couldn’t keep the orchestra quite as crisp-sounding as he had in the evening’s opening piece, Stravinsky’s “Firebird” suite (in the most famous, 1919 version), when the opening chords of the “Infernal Dance of King Kashchei” slashed taut and clean and vivid, again and again, with a sharpness of focus this orchestra doesn’t always offer. By the time the third or fourth movement of the Busoni rolled around, things were sounding a little loose in places.

Conductor Rossen Milanov charted a clear route through a varied piece that calls for a lot of turning on a dime. (Amanda Stevenson)

Yet overall, Milanov charted a clear route through a varied piece that calls for a lot of turning on a dime — as at the moment when the crashing chords at the end of the fourth-movement tarentella suddenly, and without warning, give way to a little cotton ball of sound, which ushers in some dreamier mooning around on the piano and, finally, to the men’s chorus (here, the Washington Men’s Camerata) that comes booming in in the final movement.

If you haven’t gotten the picture yet, the Busoni concerto is a kitchen-sink kind of piece, a Late Romantic behemoth — not unlike Schoenberg’s tonal and sprawling “Gurre-Lieder” — that’ is very much of its time. Musically, it’s a lot lighter than its reputation might indicate: The fourth movement, in particular, is filled with dark cartoon-villain chords yielding to passages of blithe prettiness, and the third movement features a lot of dense writing for the piano soloist that’s almost obscured beneath a thick veneer of melody in the orchestra.

The work’s difficulty is supposed to be the reason that it’s not done more often. (The NSO programmed it once before, in November and December of 1943.) But another reason might be that there’s not enough payoff; the piece breaks a lot of ground, but not that much of it is really new.

It does, however, involve some beguiling music. This is good news for a lot of the people who have the endurance to sit through something this long. If you like this kind of thing, this is very much the kind of thing you’ll like, and Ohlsson brings a lot of class to a whiz-bang performance.

The program will be repeated Friday and Saturday night at 8.