from A to B结构，A和B是并列成分，此句中from后是名词短语，to后是动词+that从句，故不是from … to …，而是to do不定式结构。进而可知是非谓语成分，找主干时可以直接划掉，就剩下 At length I returned from two weeks leave of absence. 如果知道return from短语，那句意就不难理解了。