Complex sentence: While still in his 20s, Uemura had made climbed solo Mount Kilimanjaro, Aconcagua, Mont Blanc and the Matterhorn, he had walked the length of Japan and was a member of the first Japanese expedition to climb Mount Everest.
Split into simpler sentences: While still in his 20s, Uemura had soloed Mount Kilimanjaro, Aconcagua, Mont Blanc and the Matterhorn. He was a member of the first Japanese expedition to climb Mount Everest.