Fittingly, for one of Britain's best-known, most feted singer-songwriters, the occasion will be marked by a special concert at London's Theatre Royal Drury Lane on December 7.
"I was going to do it on my own but somebody said, why not invite a few friends?" he explains. "I would only invite people who have been a great influence, or a great friend to me. It won't be a glittering occasion. It might not even be glimmering ..."
Long before then, however, there's the small matter of an 11-date tour starting next Wednesday. Reflecting its title of Celtic Cousins (an album will be on sale at the shows, and will go on general release in June), it will encompass three Scottish dates early next month, as well as Truro in Cornwall, Wales, Ireland and Northern Ireland.
"It has been at the back of my mind for a while," says McTell. "For my sins, I never worked in Ireland as much as I would have liked to. But of course I've made regular visits to Scotland and Wales over the years.
"My son Tom actually said, why don't we do a tour of these places? The idea came almost immediately to call it Celtic Cousins, and to stretch my tenuous connections with a themed album and a tour.
"It's not about me attempting to be Celtic," he adds. "It's more about me drawing on songs that have a connection with the Celtic nations, through people I've met over the years."
The set list will change, depending on the location.
"My friends in Scotland will know most of my material, so I'll probably keep the set-list fairly standard for Scotland and Wales. It might be a little bit different in Ireland, [so I'll play] hits like From Clare To Here, and Streets Of London, things like that.
"In fact, this - changing the set around - was something that Billy Connolly once spoke to me about. We both change our sets around. He said you have to - you need that little edge of danger when you're on stage. Soloists do need that, to keep from becoming a mere jukebox." He laughs. "Or a joke-box."
Connolly and McTell, it transpires, go back a long way. Twenty years ago, Connolly asked him to contribute some of the music for his BBC series, World Tour Of Scotland. One of the songs from it, The Tangle O' The Isles, has made its way onto the Celtic Cousins album.
I mention that Connolly has returned to his musical roots by playing the autoharp on a new album of folk songs by Northern Irish songwriter and music producer Phil Coulter.
"I think that's great," McTell enthuses. "He's absolutely steeped in folk music. It was our first common bond. He used to phone me up from Los Angeles and play me a banjo tune, and we'd say, oh yeah, we must get that one together when you come over."
The 11-date Celtic tour will, as ever, be just one man and his guitar(s) - and, of course, a changing selection from that remarkable McTell catalogue of 300-plus songs, the earliest of which graced his debut album, Eight Frames a Second, back in 1968.
"I always play solo," he says when we ask if he will have a band with him. "That's the way I work. I like to talk to the audience. I might have a piano with me in Scotland, but I'm not quite sure. I'm taking an electric piano, but I'm such a big chap that when I'm behind it I look like Sooty and his xylophone. I like a piano that's more my size, like a grand."
What he does know for certain, however, is that he will be doing fewer dates in future. "Next year I will truthfully be able to say I have been touring for 50 years. I do love touring. My crew are a third my age and I can go the distance with these guys, but that's because I get an adrenalin rush every night.
"People sometimes say to me, We haven't been to see you lately because you haven't been near us, and I think, you're going to have to get used to that, because I'm not going to be doing so many gigs in future.
"I like to keep it fresh. I love to play, and want to continue. I don't see a point where I will stop playing; I just see a point where I cover the entire map with dates."
McTell has had an eventful career since the mid-1960s. This was the era when, in his own words, he sought to unlock the meaning of life through poetry, art, jazz, Woody Guthrie, Blind Boy Fuller, John Steinbeck, Jack Kerouac, and hitchhiking around Europe with his guitar. He even replaced his real surname, May, with McTell, in tribute to bluesman Blind Willie McTell, "and because it needed another syllable."
Later, he was part of the remarkable vanguard of literate singer-songwriters who, in the words of Rob Young, author of the magisterial survey Electric Eden, "provided a significant fraction of pop music's core repertoire in the late 20th century". Alongside him were such names as Roy Harper, Donovan, John Martyn and Van Morrison. "I've had a fantastic career," he says, simply.
His own memorial might well be Streets Of London, his greatest commercial success, and a song with "hundreds and hundreds" of cover versions. It has overshadowed some of his other songs, but he remains "extremely grateful" that he wrote it, and it has been hugely lucrative.
A few years ago McTell marked Bob Dylan's 70th birthday with a download-only tribute of covers. Who would he pick for an album marking his own 70th?
"I'm flattered when anyone covers my songs," he says, but among the singers he most admires are Eddi Reader and Christy Moore. He laughs again. "It would be incredibly ironic if Bob Dylan were do to a Ralph McTell song, but I believe Bruce Springsteen once did a couple of versions of Streets Of London on tour."
Dylan and Springsteen on a Ralph McTell tribute album? Now there's a tempting thought. Let's start the online petition now.
Ralph McTell is playing Edinburgh Queens Hall (May 2); Glasgow SECC Lomond Auditorium (May 3) and Ayr Gaiety Theatre (May 4). www.ralphmctell.co.uk