Original: In 1875, at the age of 70 and after the death of his first wife in 1870 and his marriage to Susannah Grace Sanger in 1872, Müller began a 17 year period of missionary travel.
Simplified: In 1872, after the death of his first wife in 1870, he got married to Susannah Grace Sanger. In 1875, at the age of 70, he began a 17 year period of missionary travel.