Some more thoughts on Hashirama:-
We have seen 4 types of sage mode so far. The Toads, Juugo's Clan's, The Sankes/dragon style, and then Hashirama's. I can't say for certain how the Snake/dragon style works, whether or not it permits the user to continuously gather Natural Energy while moving.
Where I am going is this, we cannot discount the importance of Hashirama's genes/cells in his Mokuton and life force abilities. As examples, Hashirama's cells allow a none Uchiha to use the Sharingan as well as or even better than an Uchiha, and Hashirama's cells can tranform a Superior Sharingan (EMS) into a Rinnegan.
So even if Hashirama uses Natural energy, he too like Juugo may have some genetically endowed affinity to gathering Natural Energy, that's why his cells can do all kinds or wonders (LOL).
As I see it there are two options:
1) Either Hashirama's Cells allow him to gather Insanely Vast amounts of Natural energy without turning into stone, or transforming into monster like features like Juugo and Kabuto. Hashirama remains physically and mentally unchanged, except for the markings on his face. This could mean that his body is able to use Natural Energy naturally and directly, Just like the Juubi.
2) Hashirama's Chakra Reserves are Beastly, and so incredibly great, and can balance out any amount of Natural Energy without ever falling behind, while his body cells automatically gather the Natural Energy and balance it with his Chakra, without him having to concentrate on it. In other words, because of his genes the process of assimilating natural energy is automatic.
The one thing I am not sure about is whether Hashirama had to learn the Mokuton Technique, or whether his body does it naturally and automatically. I am almost convinced he does it automatically since it seems to manifest with everybody who has ever stolen or used his cells. However, like Mutants in the X-Men, Mokuton probably didn't kick in until he was at the age of puberty.