The Japanese Navy, and Army too, had been preparing for a war with America for the best part of 30 years by 1941, predating and independent of changes in strategic calculus the breaking of the UK-Japan alliance and later Tri-Partite Pact caused.

So yes Japan was prepared to fight the US alone if that's what happened, and if needed the European colonial powers like Britain, France, and the Netherlands as they controlled the resource rich islands of SE Asia that could greatly improve Japan's dependency on foreign controlled imports. While the fighting qualities of the mongrel, decadent, divided Americans was not held in the highest regard as compared to the mighty Yamato Race and its glorious cause to cast out the imperialists and lead the East Asian Co-Prosperity Sphere. At least that was what some of the more bellicose rhetoric pushed, while officers who had traveled, studied, and been posted as attaches in America, like Yamamoto, often had a healthy appreciation for the industrial power present in America to eventually simply produce a force large enough to grind Japan to dust. But the hope was that the social, ethnic, and political disunity would allow for a war that went sufficiently bad and cost enough to force America to the negotiating table before that happened. And while planners acknowledged it would at best have numerical parity temporarily with the US, especially in Naval power, if it could utilize bold tactics like night torpedo attacks, aggressive and over armed cruisers, individual ships of overpowering strength(see Yamato), and crash building programs following the breakdown of the treaty system it was hopefully going to be enough.

And this is reflected in the IJN's strategy for war. A lightning strike South into Dutch East Asia to secure needed sources of raw materials like Oil and Rubber to keep the war industry going, combined with seizing select island bases like Rabaul, Wake Island, and the Gilberts and Marshall's. The idea being to construct concentric rings of fortified island bases, all within range to mutually support each other with aircraft and light naval forces so an attack on one would draw support from surrounding bases. The attack on Pearl Harbor then was an attempt to damage, and paralyze the US Pacific Fleet to allow for these other objective to be accomplished.

From there a US counter attack would have to fight off the swarming defenders of whatever atoll they selected to attack or come near. While subs, and light surface forces would prick at the enemy or raid supply lines. Then if the enemy broke through the line of resistance would simply fall back to the next island chain. And once the enemy had advanced beyond easy reach of their home bases, perhaps into the Mariana's, and the US bloodied and worn, a decisive battle would be sought. At least the conventional idea among the orthodox circles of the IJN before the war was informed on the theories of Admiral Mahan that placed the destruction of its opposite as the key goal of any concentrated battle fleet. Light forces would strike and disorder the enemy by night attacks, while aircraft would track and harass them, to be finished by the guns of the battle line. While the basic idea would be altered by degrees over the years, it was never really thrown away all together. The fundamental hope always being that if a bad enough defeat could spill enough American blood the government would be forced to the negotiating table, and Japan could secure its newly expanded empire.