The event on 27 May offers 46- or 95-mile routes snaking through the scenic Trossachs and the Campsie Fells north of Glasgow.
The ride will raise money to develop young Scottish cycling talent acknowledge the fabulous contribution made to Scottish and British cycling by Millar, who was born in Glasgow in 1958.
His fourth-placed finish in the 1984 Tour de France was a record for a British rider and has since been matched only by Bradley Wiggins in 2009.
For three of the organisers, the sportive is the perfect way to celebrate Millar's achievements. They hope it will safeguard his legacy.
Billy Bilsland, Brian Smith and David Lang are respected figures in the Scottish cycling scene.
Millar was a pupil of Bilsland's at his circuit training classes in Glasgow in the late 1970s.
A veteran of the professional continental racing circuit, Bilsland and fellow Glasgow Wheelers stalwart Arthur Campbell helped nurture the fiercely ambitious Millar and set him on his way to France.
Smith, now manager of the Endura racing team, trained with Millar when his hero returned home from racing across mainland Europe.
He picked up the Scots pro baton that was handed from Bilsland to Millar and raced in the same Motorola team as Lance Armstrong.
He is a trusted friend of the enigmatic Scot - and a cycling nut.
"Didn't I see you win the Glasgow-Dunoon race in the '80s?" I wondered.
Smith, from Paisley and now based in Kent, replied that he had indeed won, aged 18 and at his first attempt - and he could even remember the gear combination he used in the final 25-mile section.
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Lang, who captains Millar's former club, the Glasgow Wheelers, said: "In my opinion, he is definitely the best stage rider that Britain has ever produced.
"Yet I'm sure if you stopped most people in the street and asked them who Robert Millar is, most Glaswegians wouldn't know.
"So we looked at the possibility of setting up a sportive in Robert's honour.
"He did a lot of his training over the Crow Road, between Lennoxtown and Fintry, and that is the final climb.
"The circuit also goes over the Tak Me Doon from Kilsyth, which is tough as well."
Millar's steely determination and aggressive climbing skills saw him finish fourth in the 1984 Tour de France, as well as securing second places in the Tours of Spain (twice) and Italy.
And then there were apparent contradictions: self-deprecating and shy or self-assured; pony-tailed waif or tough as teak; proud of his roots or occasionally dismissive - "the Caledonian antisyzygy" on two wheels.
Lang put it thus: "Robert was very much a loner. He was determined to be a world-class cyclist and it didn't really matter who he upset to get there."
Bilsland, to whom Millar gifted his 1984 King of the Mountains polka dot jersey which now hangs on the wall of his bike shop, added: "Robert was special.
"He was focused on what he was about. He believed in himself. When he attacked, he attacked to win.
"I think he was one of the few Scotsmen who ever got their photograph on the front cover of L'Equipe.
"I believe if he had the backing of the Sky team now, he would have won the Tour.
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"When he was with Peugeot, a lot of the French guys didn't want to ride for him. A lot of the victories he got on his own merit."
The sportive, Lang believes, is the first time Millar has allowed his name to be used for a cycling event.
The intensely private Millar gave his permission to Smith as long as proceeds went to the Glasgow Wheelers' development squad for young riders and the Braveheart Fund, which Smith established a decade ago to improve young Scottish cyclists' prospects.
"He was one of my idols," said Smith. "He used to come back in the winter to Glasgow. Every year he came round to the house and gave me a jersey.
"I was fortunate then; I was just as small as him so I could use them.
"And now, to have a Robert Millar Sportive is fantastic because the legacy of the name will go on.
"Every hill that you go over will remind you how difficult it was to become King of the Mountains in the biggest annual sporting event in the world."
A generation after Robert Millar mesmerised British TV viewers with his assaults on Pyrenean peaks in mid-1980s Tours de France, a sportive in his honour will test riders on the roads on which he trained.