The Chip Ganassi Racing-Honda driver finished the 75min Friday practice session 5.4sec off the ultimate pace around the bumpy 2.35-mile parkland course.
Johnson, who hasn’t raced since the Grand Prix of Indianapolis on the IMS road course a month ago, said: “Thankfully I tested at Road America last week so I kinda knocked the rust off there.
“But I’ve done a lot of sim work at home and at HPD and nothing got me ready for the experience here!
“I’m just floored by how technical this track is, how rough it is, how little grip there is. I have a huge challenge ahead of myself this weekend.”
Regarding his spin in practice, he said: “I’m just trying to find the limits of the car and of my ability.
“I just got into Turn 3 a little too fast, had a little too much rear brake in the car, and did a little lazy spin. It’s just one of those things that rookies do, and I’ll keep learning from those mistakes and get better.”
The mid-80degF heat and high humidity are expected to prove difficult for the drivers, given the aeroscreen, despite IndyCar mandating the air vents on top of the rim of the screen.
“The heat wasn’t bad, just for an hour of practice,” said Johnson. “The implications of the heat are really going to show up on Sunday, mid-race.
“We’re used to it for one race, so Saturday, we’re prepared for that. It’s getting recovered and ready for Sunday that’s going to be tricky.”