It would not be a surprise if Johnstone was still struggling to grasp what had happened in Rio.
The New Zealand team had got itself into gold medal contention but finished in fourth place after its most experienced and decorated rider, Sir Mark Todd, knocked over four rails in the showjumping round on his mount, Leonidas II.
''It would have been nice to come home with a medal but I'm just pleased with the campaign, really,'' Johnstone said.
''A lot of it comes down to experience and I'm feeling good about sixth place. I'm just really looking to build on that for the world championships and the Olympics next time around.''
Johnstone, who arrived home in New Zealand this week, was left as the country's highest-placed rider following Todd's calamitous run.
The 29-year-old was born in Dunedin and grew up in Outram. He was competing at his first Olympics and has at least another ''couple of chances''.
If he carries on riding as long as Todd, he could compete in another eight Olympics, he joked.
But the next big event is the world championships in 2018. Before that there are plenty of individual events to keep Johnstone busy.
He plans to have a break for a few weeks. After that he has another 10 horses in his stable to keep working.
Balmoral Sensation can expect to put his four legs up a little longer, though.
''He had a huge year this year, competing at Badminton and the Olympics, so he will have a pretty light year next year.''
It will give Johnstone an opportunity to put some more work into several horses ''with top-level potential''.
''A horse called In Disguise and one called Do Volente are both two-star level but both are very, very nice horses for the future.''
Balmoral Sensation turned 12 recently but potentially has another Olympic campaign ahead.
''Horses are usually in their prime from the age of 10 to 16. Some go on until they are 18. But if I look after him well, and don't do too many competitions, he should certainly last until he is 16.''
Johnstone will return to Dunedin next week and will visit his old primary school - Outram School - on Monday.
He will also be opening the new clubrooms at West Taieri Pony Club where he got his start in the sport.
