But how do you attack such a formidable obstacle? Willis has a new strategy, modified with the knowledge of what didn't work in Glasgow.
"This time, I'm going to have a more balanced emphasis on my training," Willis told Radio Sport.
"When I attempted it in Glasgow, I really hadn't done the 5k training, I just tried to rely on my natural strength and use my 1500m fitness, and I was hoping it would be a slow race so I could use my speed at the end.
"This time, I'm going to have a lot more endurance behind me and hopefully haven't lost too much speed.
"It's an exciting endeavour we're embarking on to give the 5k a real serious crack, and hopefully my 1500 will only benefit from that as well.
"It's a bit of a headache but after [the training sessions], you feel on top of the world, you go 'wow, I can take on anything after getting through that'."
First, Willis will need to continue to rediscover his best form after a frustrating year.
"I had some big hopes but they got derailed with some injuries, and some doubts crept into my mind — maybe it's not the injuries at all, maybe it's just me getting old, maybe I won't be able to re-find that form which got me to a medal in Rio.
"Fortunately right at the very end, I was able to put together a complete race [at the Fifth Avenue Mile]," he said.
"Patience is the main thing I'll take away from 2017 — to trust my training and not get too disillusioned after not getting the results I want in each race."
Despite the brief doubts, Willis remains confident that, aged 34, he's not past his best just yet.
"It's got to intimidate my competitors when they see the greys in my hairs," he joked.
"We've seen it in many sports in the past few years — our understanding of what is the age limit is really evolving as it becomes more professionalised and people aren't relying on income outside of their sport, so we're able to keep going a bit longer."
Willis has several indoor races and an altitude training programme in Arizona to tackle first before the Games arrive — a challenging build-up to one of the most challenging quests of his career.