The Briton will hand over the MCL40 to Australian teammate Oscar Piastri for Thursday's session, before getting back in on Friday (local time) for the team's third and final day at the Circuit de Catalunya.
"It was just nice to be back, nice to see a number one on my car, pretty cool and pretty surreal still. It's a good feeling," said the 26-year-old after lapping in cold temperatures at the Spanish Grand Prix track on Wednesday (local time).
"Our first day on track, first time everyone gets to see the car in one piece.
"It's literally not been built until this morning ... nice to see it all come together.
"And then I go and have some fun driving it, which turned out to be a decent day.
"Today was really just a first understanding of the whole car, understanding just how it works really, going through the manual."
The five-day test, which started with closed sessions on Monday, allows teams to choose which of three days they run on. Williams are the only team absent.
Mercedes, who provide engines to McLaren, also made positive noises after completing significant mileage with George Russell - already considered one of the favourites for the season - and Kimi Antonelli alternating.
The team had opted not to run on Tuesday because of bad weather.
Russell completed 92 laps in the morning, taking his tally so far to 187, and said everything had gone "relatively smoothly" as far as reliability.
Antonelli ran a similar program in the afternoon and did 91 laps for a two-day total of 147.
"We are pleased with the mileage we achieved today and have ticked off most of the objectives we set ourselves for the first two days of running here in Barcelona," said trackside engineering director Andrew Shovlin.
"The car has been reliable and we've been able to put together multiple race distances today. That is encouraging as an initial step."
Mercedes-powered Alpine completed 125 laps, with Pierre Gasly doing 67 in the afternoon.
Haas reported two reliability issues, the second unspecified one causing more concern, with Ollie Bearman in the Ferrari-powered car.
"I'm not worried about the one we had this morning, the one in the afternoon was a bit more serious and it's obviously something we're looking into and analysing exactly what happened," said team boss Ayao Komatsu.
Audi, the former Sauber team now using their own engine, also lost time in the morning with a technical issue.
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