Willis has struggled to reach peak fitness for the event after suffering a calf strain in a fall at a Diamond League race at Shanghai in May.
He and wife Sierra have also become parents for the first time to son Lachlan who was born on July 4.
Willis' brother Steve who helped the 30-year-old prepare for his silver medal at the Beijing Olympics gave him an analogy to think about before the race in reference to his abridged preparation.
"He said it's like when you put a cake in the oven and it sits there and sits there before it suddenly rises. It almost did; I'm not that upset at my performance.
"The injury has been the main hindrance on my campaign. I didn't run a step for four weeks, nor could I cross train, other than swimming with my arms. It was the muscle at the top of my calf so even if you take the weight off you're still often contracting it. I did my first proper workout the day before my son was born."
Willis was contented to have spent valuable time at home.
"I'm loving being in love again. I watch videos of him and can't take the smile off my face.
"I'm trying to help out but I sleep through the crying. My wife has to shake me awake to change the diapers. I'm happy to do everything I can but really there's only so much a man can do. It's pretty arduous for my wife but she's been extremely supportive so I haven't lost any sleep in preparation, except when I left and came here. Maybe I was so nervous because I haven't raced much for so long. I could only relax once I got that heat under my belt."
Willis took the 24th and last place in the semi-final field as one of the six fastest qualifiers once six came from each heat.
"I felt a lot better today. I'm not sure whether it was nerves or coming over too late but I was so run down before the preliminary. I slept about 90 minutes before that first race.
"After two good nights' sleep I felt a million bucks today, relative to where my training was at. I'm not that disappointed to be honest. At least I was competitive rather than slipping off the back."
Willis will now race in Europe over the next month.
Fellow New Zealander Zane Robertson finished 14th in the 5000m. His time of 13m 46.55s was almost 20s behind the winner, Brit Mo Farah. Farah completed the 5000m/10,000m double at the meet, just as he did at last year's Olympics.
Robertson held on to the main group until the final two laps.
