Saint John's Cem Clay Co TN Death date of 1880 on her tombstone must be wrong if she was still living in the 1900 census.
She is married to William Canada (Kennedy) Long.
They got married on October 21, 1872 at Clay County, Tennessee, USA, she was 28 years old.Source 1
I did not put William Canada Long's (1862-1947) mother as Martha Dulworth because the birth took place ten years before the marriage. On the other hand, Martha Dulworth Long indicates on the 1900 census that she has given birth to one child with one child still living. If someone has more information on this please contact me.