However, the writer/actor/producer, who in the past 10 years has achieved worldwide success with the Evening Times-backed, A Play, A Pie and A Pint lunchtime theatre at Oran Mor in Glasgow's West End, is suffering from Motor Neurone Disease.
He was given the 'catastrophic diagnosis' by his consultant a year ago.
But the fun in the 65-year-old, highlighted by one Oran Mor play appearance wearing nothing but a giant nappy and a pleased expression, is far from gone.
"I've been remarkably lucky," he says of his life.
"I think one of the things about getting this wretched illness is it focuses the mind on how lucky I've been with my work, my family and my friends.
"Until I got this I wasn't quite as aware as I should have been.
"Then when you get what my neurologist called 'a catastrophic diagnosis' - he wasn't mincing his words - you've got to ask: 'Will this define me?'
"At some point I guess it will, but for the moment, I'm going to carry on."
He is the man who has enticed Robbie Coltrane, Elaine C. Smith and Bill Paterson to star in plays in his Oran Mor basement theatre. MacLennan, a hugely colourful figure and the son of two Glasgow doctors, who ignored his parents' advice not to pursue a career in theatre, has chosen not to go public about his illness until now.
Yet, MacLennan, who is married to actress and Balamory star Juliet Cadzow, reveals he's won't lie down to the illness, and says he is determined to carry on working.
And although he is slowing down, the positivity which saw him once troop his troupe around the Highlands and Islands in rickety vans playing to tiny audiences - and sometimes free range animals - hasn't left him.
In the past 10 years however his A Play, A Pie and A Pint series has not only seen audiences soar, the success story is being replicated in Edinburgh, Ayr and Aberdeen.
Co-productions now operate throughout the UK, and the winning formula has been franchised to Philadelphia, with Caracas and Sao Paulo due to follow soon.
Oran Mor writers' efforts have been translated into Italian and Russian, performed in New York, Moscow, Adelaide and, later this year, Paris.
And thanks to Dave MacLennan and Co, bite-sized chunks of Scottish culture are being served up with pies (or pizzas in Philly) around the planet.
But how to explain the astonishing success of this colourful character, (his costume today is sunburst cords and a fire engine-red body warmer)?
It transpires it's down to the man's love of theatre.
"I've loved it since I was around six-years-old and my parents took me to the King's Theatre to see Peter Pan," he recalls.
"As the little Tinkerbell light faded, Peter Pan came forward and called out; 'Boys and girls, Tinkerbell is dying! But we can save her. I want you all to stand up and shout out: 'I believe in fairies.'
"Before I knew it, I was standing on my seat shouting at the top of my voice. And I've been captivated by the magic ever since."
Over the years, Dave's doctor parents, (his father was the eminent obstetrician Sir Hector MacLennan) drip-fed their son with a range of theatre, from high drama to variety.
"Jimmy Logan moved in next door and we'd get invited, en famille, to the Alhambra Theatre," he says.
"I'd visit Jimmy in his dressing room, with all the wonderful mirrors, costumes and make-up boxes then go backstage and see the Bluebell Girls dancers close up, with their legs all the way up to their oxters.
"It was lovely."
The boy David made his stage debut at boarding school, in Charley's Aunt, playing Lord Fancourt Babberley - in drag. He'd fallen in love with acting.
"I don't think my parents fell in love with the idea, though," he says.
"They would have been happier with a stethoscope around my neck as opposed to a string of beads. But unfortunately, as an actor I was second division.
"Wildcat lasted 20 years.
"We never got rich, but we never stopped touring. I'm so lucky I came into the business when it was growing. It was fun, sharing attitudes and politics."
And often the back of a van?
"Oh, yes," he says, "and one night in Bo'ness village hall our play was plunged into darkness while we found 50p for the meter."
But he would have come up with a different ending for his Final Act.
"I can imagine choosing to die in the middle of the Tay, with a salmon on the line and a hip flask in my pocket, going out to a giant heart attack aged of 104," he says with a wry smile.
"But it's not to be. And you know, I've had a quite fantastic life."
And with that he's striding home for an appointment with his laptop, to write his memoirs and come up with more cracking lines and agitprop gags.
He's still smiling. Still wearing bright costumes. And he still believes in fairies.
l Motor Neurone Disease is a degenerative disorder of the cells that control voluntary muscle activity. There is no cure, however treatment can relieve symptoms and its slow progression.