If it doesn't kill you, you don't adapt.
Driving on the left is the natural thing to do (according to C. Northcote Parkinson), but it's not the way most of the world does it.
It's not at all difficult to adapt from side-to-side, as a driver or pedestrian, since the whole thing is very Darwinian. If you get it wrong you could die! Really, all it takes is about three weeks of being very conscious about what you're doing (look left, right, left again or vice versa when crossing the road) and it's pretty much ingrained.
The other social conventions that follow from the driving conventions do not seem to have embedded themselves in my little mind after five years on the other side. Much to my surprise.
Most of the time I do not walk down the wrong side of the sidewalk, but the habits of 25 years of veering left when faced with oncoming people is still with me. Last year I nearly ran down a woman with a heavy baggage cart at LA airport as I hastened between the terminals. As I had just been six weeks in left-hand drive/walk countries this was understandable.
But last Friday I was a good 18 months away from left hand drive/walk conventions and I still did the wrong thing. Ambling along the Minnesota river trails a mountain biker came round the corner, in the center of the trail, where I was running. So I veered to the left. He shouted at me. I kept on going left. Instinctively. Correctly. He skidded. The river loomed beside him. He put his hands out to save himself. I hopped to the right and managed to avoid his bike flying out from underneath him across the trail, and he got good value out of his gloves as he rolled onto the gravel. He picked himself up and swore at me. I apologized, hoping the accent would suffice as an explanation. But he was already peddling away, still swearing at me.
Watch out for me on the trails! And keep to the left right.