"It was quite hard to be isolated, just by myself. Not in the beginning, but after a few weeks it was quite tiring," Marco Weber told a news conference at Greenpeace's Zurich office on Tuesday, a day after his return to Switzerland.
Weber was one of 30 Greenpeace activists on board the Dutch-flagged ship Arctic Sunrise seized in September by Russian security forces over a protest at an offshore oil rig owned by state energy giant Gazprom.
The activists had faced lengthy prison terms before Moscow announced amnesties.
Weber said Russian authorities "were looking for a solution" in the case.
"They tried to find a way out without charging us but at the same time without saying that we were not guilty," he said.
Tired looking, the experienced 28-year-old mountaineer said he had not been treated badly in Russia but added that "the lack of contact with other human beings was very hard".
Nevertheless, Weber said he did not regret having taken part in the Greenpeace protest to protect the Arctic's unique ecosystem and "would do it again".
Weber arrived in Zurich on Monday after a 50-hour train ride from St. Petersburg.
Two days after Russian lawmakers approved the Kremlin-backed amnesty bill that ended the 30 Greenpeace crew members' prosecution on December 18, Gazprom announced it had begun oil production at the Prirazlomnaya oil rig that was the focus of the activists' actions.