Joe Root vowed he still has "plenty more left to do" after a marathon century in Multan saw him become England's record Test run-scorer.

Root needed 71 to leapfrog Sir Alastair Cook's mark of 12,472 and become the the most prolific batter in the nation's history, securing the honour in emphatic fashion on day three of the series opener against Pakistan.

He batted all day to reach 176 not out, showing deep reserves of skill and steel to steer his side to 492 for three - just 64 behind the hosts' 556.

It was more an examination of his physical toughness than his renowned technical prowess, with the 33-year-old laying siege at the crease for over eight hours and battling brutal bouts of cramping, muscle fatigue and dehydration in the unforgiving Punjabi heat.

In nine sessions, he has been off the field of play for just eight deliveries.

Root shared stands of 109, 136 and 243 with Zak Crawley, Ben Duckett and fellow centurion Harry Brook (141no) as he ground out his 35th Test ton, extending another record he inherited from Cook in the summer.

Only four of the sport's authentic greats now stand before him among all-time run-scorers - Rahul Dravid, Jacques Kallis, Ricky Ponting and Sachin Tendulkar in pole on 15,921. At 33 years old, the popular Yorkshireman is ready to keep on churning out the scores in his bid to outdo them all.

"I'm obviously proud but I still feel there's plenty more left to do and many more runs to get," he said.

"I always think your best (hundred) is your next one. You've got to keep staying hungry and willing to find ways of contributing, ways of getting better.

"I'm sure I'll be looking back on it at some point when I'm finished and be very proud of it.. but we've still got an opportunity to go and win this game which is exciting. We'll see (how it feels) at the end of the Test match."

Duckett was partnering Root when he unfurled a classical straight drive off Aamer Jamal to reel in the record, a huge landmark moment but one that passed by with only a tiny flicker of acknowledgement from the man responsible.

Root allowed himself nothing more indulgent than a glove punch with the left-hander, who patted him on the shoulder in return, and the briefest wave towards the standing ovation taking place on the away balcony.

"He was 100 per cent aware but he made out he didn't have any idea," Duckett said with a smile.

"I asked him and definitely knew. It was pretty cool being out there in the middle with him for that. He's one of the greats of the game and certainly one of the greats of English cricket.

"It doesn't always feel like that with Rooty because he's incredibly humble and these things don't bother him.

"It's just a pleasure to share a dressing room with him. We make jokes that he's out of form when he's getting Test hundreds. It's just special to have him around and to witness the form he's in at the minute."

Pakistan coach Jason Gillespie, who worked with a young Root for five years when he was in charge of Yorkshire, recalled their time together fondly.

Speaking on behalf of his team after Root's relentless run-making ran them ragged, the former Australia seamer said: "When I was at Yorkshire he'd just started his journey in first-class cricket.

"I remember Joe asking coaches, 'Tell me what I need to hear, not what you think I want to hear'. That was a coaching lesson for myself.

"He always strives to get better and always strives to improve, that was my experience with him."