"The challenge is not to be reaching the same levels, it's to go higher," Mako told BBC Radio 5 live.
"Billy has come back in and trained well, and his knee is looking good. He actually looks a lot better than me, so fair play to him."
Mako said staying on top of their diet helped the duo stay in good physical shape and also credited England's nutrition consultant Graeme Close for monitoring their progress.
"It's always a thing with me where I have to keep on top of my diet, and I am getting as much help as I can," the 26-year-old added.
"We have Close who keeps a close watch on me."
Reigning champions England top the Six Nations standings with three wins on the bounce and another on Saturday would take their run to 18, level with New Zealand who hold the tier one world record.
(Reporting by Hardik Vyas in Bengaluru; editing by Sudipto Ganguly)
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