Soviet logistics would have been shoestring, but Japanese defenses were even more lacking at the time. As /u/cckerberos points out, the attack on the Kuril Islands was difficult for the Soviets ─ and probably would have resulted in an initial Soviet defeat a la Wake Island ─ but the situation on Hokkaido would have played to Soviet strengths, as its larger area would have allowed the Soviets to use force in numbers in a way they could not in the Kurils, which offered limited landing areas and poor maneuver terrain.

Hokkaido had been all but stripped of men and supplies, and moving soldiers and equipment back to Hokkaido would not have been easy, as demonstrated by Task Force 38's strikes on July 14 and 15. These sank eight of the 12 large rail ferries used for transport to and from Honshu; the other four were unusable because of damage.

The Soviet plan for an invasion of Hokkaido was simple and intended to deploy elements of the First Far Eastern Front from Sakhalin. The lead division would seize a beachhead around a port with a single rifle regiment. The rest of that division would follow, then two more divisions would land at the captured port. From there, follow-on forces would quickly outnumber the Japanese.

Without specialized landing craft, the Soviets intended to purposely ground trawlers, freighters and other ships on beaches. The invasion of the Kurils allows a glimpse of other methods of assault ─ small boats sent from minesweepers, destroyers and trawlers.

The Soviets estimated the Japanese had 50,000 armed men on the island ─ in fact, there were only two divisions and one brigade, and they were oriented toward the eastern side of the island, against a possible American invasion. Japanese airpower was almost nonexistent, as was the Imperial Japanese Navy.

Marshal Vasilevsky, the theater commander, sought permission from Stalin on Aug. 18 for an assault on Hokkaido. When Stalin failed to reply after several days, Vasilevsky sent a second message on Aug. 20, saying he intended to attack on Aug. 22. This was likely premature, as pacifying Sakhalin would have taken probably another week. At midday Aug. 22, Stalin ordered the operation suspended indefinitely.

As Richard Frank concludes in Downfall: The End of the Imperial Japanese Empire, "The Soviet Navy's amphibious shipping resources were limited but sufficient to transport the three assault divisions in several echelons. The Red Army intended to seize the northern half of Hokkaido. If resistance proved strong, reinforcements would be deployed to aid the capture of the rest of Hokkaido. Given the size of Hokkaido, the Japanese would have been hard-pressed to move units for a concerted confrontation of the Soviet invasion. The chances of Soviet success appeared to be very good."

Tsuyoshi Hasegawa's Racing the Enemy covers the Soviet invasion of the Kuril Islands as it actually happened in some detail. He portrays it as a reckless dash by poorly prepared, trained, and equipped troops; the Soviets succeeded in capturing the islands, but the Japanese actually inflicted more casualties on the Soviets than vice versa. Hasegawa says that, had Imperial Headquarters not ultimately ordered the defending troops to surrender, the invasion would have failed (at least initially).

The Soviets would undoubtedly have fared better in a well-planned and supported operation, but I think the haphazard way things actually played out shows that Soviet plans for the invasion of Japan had not progressed very far by the time the war ended.

Incidentally, Stalin did order the invasion of Hokkaido on August 18. He reversed his orders four days later for reasons that are unclear, however.