But that was more than 40 years ago. Today, his music is everywhere. Why would the best-recognised classical composer on the planet still undertake the arduous process of touring?
"Let's not forget, one of the great joys of being a musician is playing music," he says.
Partly, too, he does it because he can. Glass attributes his good health, musical and physical, to yoga, which he has practised since his 20s.
"Look, I'm 77," he exclaims. "I'm still playing music. I tour, I practise. I don't have any choice. I still have to. Because of yoga, I've had a long, healthy, active life - without even going into the other benefits, which help to control stress, tension, anxiety and all the maledictions of contemporary life. It's a practical way of living. I doubt that you can consider working into your 90s if you haven't done it. My best yoga teacher died recently aged 120."
Did he just say "considering working into your 90s"?
"Music has now been declared free," Glass remarks, with the merest hint of acid. "It's been very hard for musicians so, mostly, people are playing again."
In 2007, he collaborated with Leonard Cohen on The Book of Longing: "He's older than me and he's out playing again. That guy stands up and does a three-hour concert. I think he's 80."
Glass credits his own rigorous training for his prolific output, which includes 10 symphonies, 28 operas and chamber operas, shedloads of concertos, chamber music, theatre music, choral works and piano pieces, about 30 original film scores, and a torrent of collaborations with the great and good of the literary, dance, pop and film worlds, among them Allen Ginsberg, Mick Jagger, Jerome Robbins and Doris Lessing.
His intense work ethic is deeply ingrained, not least because it took him so long to begin making a living out of music. After studying in Paris with the doyenne of composition professors, Nadia Boulanger, as well as working with the great Indian musician Ravi Shankar, he found himself in New York in the late 1960s with a young family (he has been married four times). To bolster their finances while trying to make headway in composition, he took a succession of jobs, including plumbing and taxi driving; he started his ensemble and, in what time remained, he delved with near obsession into the study of Gandhi in India, which eventually led him to compose his opera Satyagraha. Only after its success was he able to devote himself entirely to music.
"I was 41 by then. Even the year before, I had no idea that later I wouldn't be working at a day job. My cab licence came up for renewal and I renewed it. I had no confidence that I would be able to make a living."
He also decided to be his own publisher: "Otherwise I wouldn't get the income. At the beginning I made most of my living playing and I wasn't going to give the material to somebody else. If you want to have a retrospective concert then you have to hire my group. That's still true today."
One of Glass' most quoted comments is: "When society becomes unhinged, the arts get really good." Now this is truer than ever, he suggests. "Today, the arts are getting really good. There's an idealism in the people under 30 that I haven't seen in decades. No one's thinking about careers or money; they're just trying to make work, writing music and playing concerts in small places. That was how it was when I was beginning in the 60s. It's a vocation, a calling, and they have a passion for it that I haven't seen in a while. I do think when the ship of state is sinking, the ship of art is going up."
The label "minimalism" belonged to music he wrote decades ago, Glass emphasises, before his ground-breaking early opera Einstein on the Beach, first performed in 1976. Early pieces feature in the Glass Ensemble's tour, but its spectrum of works is impressive. "There'll be music from three or four different decades. It does all sound like me, but it doesn't all sound like minimalism. Some of those old pieces are still very playable - The Photographer or Einstein - but there'll be more recent pieces and some of the film music.
"People often think I've written 50 or 60 film scores, but actually it's about 30."
It is easy to overestimate the amount Glass has written for the movies because his music has become so ubiquitous; and when he compares his work to that of other creators, he looks less to niche spheres of contemporary music than to colleagues and collaborators in pop and related areas.
"I'm not having the Paul Simon or Lou Reed kind of sales. The people in the pop music world I know make a lot more than I do. I've usually been two decimal points away from them. If I sold 1000 records, they'd sell 100,000 and if they sold five million I could maybe sell 50,000, and that would be a lot. The first time I sold a record and got 1000 sales, that was astonishing - I think it was Glassworks, which was a really popular piece."
But, with the increasing problem of cheap internet streaming or, worse, free access, which is damaging incomes across the musical world, he is keen to hand on his hard-won practical view to younger composers.
"When I have young assistants who write, I'm making sure they know how to take care of the music, because you don't know when you're going to get your hit. There was the Beatles' first album, which sold for a ridiculously low amount of money but, mostly in the pop music world, they're more smart about that. In the world of art music, concert music and opera a lot of the voices are still not willing to think about the business part of it." Independent
