The saxophonist, who has led his own trio, quartet and sextet and who is one quarter of the acclaimed horn ensemble Brass Jaw, had played in jazz orchestras since the age of 15 and has featured as a star soloist and arranger with the Scottish National Jazz Orchestra.
Standing in front of 16 musicians and being responsible for producing the repertoire that will enable them to appear in public was still quite a step, however. Yet it's as nothing compared to the task that Towndrow has set himself with Pro-Am, the suite of music he's been commissioned to write as part of Creative Scotland's 20 For 14 series, which forms part of the cultural supporting programme to the Commonwealth Games in Glasgow.
The Pro-Am project involves composing music for both the Byres Road ensemble (the "Am", since the band is comprised of amateur players) and the Ryan Quigley Big Band (which represents the "Pro" part of the title, being a band of top professional musicians led by Towndrow's Brass Jaw colleague, Glasgow-born trumpeter Ryan Quigley, whose recent work has included tours with Robbie Williams and George Michael and dates with Holland's prestigious Metropole Orkest).
"It's a fantastic opportunity," says Towndrow. "Having two big bands onstage together isn't something that happens very often and of course it's a challenge to write for two jazz orchestras, especially writing for two rhythm sections - having to come up with parts for two bass players has actually been one of the most taxing aspects.
''But I'm really interested in exploring the nature of professional and amateur endeavour and how these two units become cogs in the same machine, working together, because the stage is a great leveller, as I'd imagine the sports field would be.
''Once you're out there, your background and training become irrelevant. The only thing that exists is the person and it's about what he or she can do in that moment."
Pro-Am may denote teamwork between these two bands with very different ranges of experience but it also might be interpreted as being 'for the amateur', as Towndrow explains.
"The commonly held impression is that an amateur is someone who does something not very well for no money," he says. "But if you look at the root of the word, it comes from old French and means 'playing for love' and for me that's a much better starting point.
''It doesn't preclude quality or commitment and it's been great to watch the thrill that musicians in the Byres Road Big Band get when they've worked on something and taken it out in front of an audience. As a professional musician, you can take that sort of experience for granted because you're doing it all the time, although you never lose the buzz of live performance."
Quality and commitment were two factors that Towndrow noticed recurring when he began his research into athletes to see if he could take something from their stories as inspiration for the Pro-Am suite.
One of his favourite examples is Scottish boxer Ken Buchanan, who was the ABA Featherweight Champion before turning professional and becoming the undisputed World Lightweight Champion. He's also credited as one of the pioneers of using Scotland The Brave as a national anthem at sports events, and although Towndrow won't be quoting from that tune, he does have something anthemic lined up for the Buchanan sequence.
Just as Towndrow began putting his thoughts together on Pro-Am, it was announced that Robert De Niro is to star in a film that captures one of the less glorious moments of Buchanan's career, in which he lost a fight as the result of what's euphemistically referred to as a 'low blow' from Panamanian Roberto Durán at Madison Square Garden in New York.
Another of Towndrow's inspirations was also to prove topical, but in an unfortunately more tragic way. The Canadian athlete Olga Kotelko didn't take up track and field events until she was 77. A former teacher, she had hardly any sports experience, beyond playing baseball in her youth, when she retired in 1984.
She began to play slow pitch softball and, having developed running and throwing skills, she decided to take them further, found a trainer and began competing in long jump, triple jump, high jump, shot put, discus, javelin, the 100 metres, 200 metres, 400 metres and 4 x 100 metre relay sprints. Not just competing, in fact, but winning: she amassed 750 gold medals and held 30 world records in her age range, and her physiology and muscle tissue became subjects of studies by some of Canada's finest medical brains.
Sadly, Kotelko died, aged 95, last month and on hearing this news, Towndrow abandoned the piece he was writing for her, which he had struggled with and felt didn't do her justice; he has now written a new section of the suite dedicated to her memory. He has been in touch with her family and has received positive feedback from her daughter Lynda.
"When I started working on the suite, I wanted each of the movements to be inspired by a sporting story or a sporting character who embodies our idea of greatness, yet is perhaps less well known in the public consciousness," says Towndrow. "Ken Buchanan's still a boxing hero in Scotland and his story would still be inspiring, with or without the Robert De Niro angle, just in terms of what he achieved.
''But although Olga Kotelko carried the torch at the Winter Olympics in Vancouver in 2010, she's maybe not that well known over here and she absolutely fits the idea that behind the public face and actions of athletes and sportspersons there's another layer, a deeper narrative. I found it remarkable that she achieved so much - she's now regarded as one of the greatest athletes of all time - but the more you look at it, the less relevant this notion of greatness becomes to the effort of human endeavour. They just get out there and do it."
From a practical viewpoint, Pro-Am has given Towndrow decisions to make, such as how to line up two jazz orchestras on stage. His ideal is to have one band creating the mirror image of the other, with the lower-voiced horns on the wings and the rhythm sections occupying the middle ground, but that may change once the stage is set.
"I think all along the way there's been an element of having to do it to find out how it's done," he says. "With the music there was the question of should I take the raw components and see how they might sit with a single big band then expand them or try a different way? But I can now hear in my head what it's going to sound like and that's made it easier to score the parts."
Opportunities to repeat the exercise are likely to be thin on the ground, although Olga Kotelko achieved some of her biggest triumphs at the World Association of Veteran Athletes World Championships in Gateshead, and it hosts one of the UK's most forward-looking, most ambitious jazz festivals. For the moment, however, Towndrow is looking no further than the premiere and the chance for his Byres Road Big Band charges to play on a major concert platform.
"It's been interesting watching the band develop because there are musicians with different ambitions involved," he says. "Some of the older players have had successful careers outside of music and are doing it for the love of it; others might well follow our two former members, Corrie Dick and Pete Johnstone, who went on to become Young Scottish Jazz Musicians of the Year and are now working in the professional music world.
''But it's great for them to be part of something like this, something so unusual, and that's the value, for me, of the 20 For 14 project: it's giving musicians and composers the opportunity to think big and create music that could make a lasting impression as part of this big sporting occasion."