2014 Moto3 runner-up Jack Miller was left wide-eyed and amazed after his MotoGP debut on LCR Honda's Open class racer.

The Australian put in a mammoth day of testing work at Valencia, lapping the Ricardo Tormo circuit 71 times in just a five hour window. According to Miller the power of the machine's brakes took a lot of getting used to.

"I went out of pit lane and went in to corners one and two," he said. "I was like, 'Shit! That corner came quickly!' Everything comes up so fast. It's so hard to describe, it's out of this world.

"The speed of course was a shock but after two or three exits you get used to it. But nothing can prepare you for the brakes. It's an incredible feeling. All they want to do is tear the bike out from underneath you.

"It's an amazing bike to ride and I've had the best day of my life."

Although he appeared upset on the Moto3 podium on Sunday morning, having lost the championship by just two points, the thought of making his debut on the MotoGP machine kept him awake come night time.

"I didn't sleep a wink! The anticipation, I couldn't wait for twelve o'clock to come around and as soon as we got on it we kept cutting laps. We didn't stop! We did 71 laps and we weren't so far away. We were really happy with where we were.

"We weren't trying for lap times today. I really came at one with the bike and got a great feeling of it. Setting wise we made some adjustments that we weren't planning on making today.

"We didn't quite get to try the harder option front tyre, which would have been nice because we seem to be struggling a little bit with the softer one. You could feel it squishing a bit too much under brakes. The track temperature was going down too much this afternoon so we'll wait again for tomorrow and we'll be back on it again."

Miller finished the day 22nd overall, 3.6 seconds slower than Jorge Lorenzo's fastest time of the day. Yet that was nearly some five seconds quicker than his best time aboard his KTM Moto3 machine.

"The power was a massive improvement. It's so difficult to describe the difference in going from 60 horsepower to 130 something like we have now. It was a big step to overcome and now we need the braking. I've never ridden a 600. The Moto3 was the biggest bike I had ridden until today.

"Of course it was a little difficult to change direction and everything happens a little slower. It's a special bike. Earlier in the day I was trying to ride it like a Moto3 and ride it smooth but you can't do that at all. You have to do a 'V', stop it and turn it on the rear tyre."

Like fellow class rookie Maverick Vinales, Miller completed the day without traction control or anti-wheelie in a bid to get accustomed to the extra power. The Australian was also happy with his performance on worn tyres.

"It's not so bad really, not as bad as I expected. The tyres worked well all day and I was able to get within 0.4s of my best time on really used tyres so I'm happy for that."

That increase in power and the gruelling lap count left the 19-year exhausted and he revealed he has plans to alter his winter training regime.

"Of course I'm tired. We did 71 laps. We need just more or less bike time to get my strength on the brakes. We're working on getting a trainer and we're starting a new programme. We had to wait for the Moto3 season to be over before we can do it. Building muscle isn't what we are aiming for. We just need to use the muscle we have better so it's lean."

Testing at Valencia continues for the next two days. Miller, like the other Open class Honda riders, will race the upgraded RC213V-RS machine in 2015.