The word "flesh" here is a metonym for the body, and "living in the flesh" is a metonym for being alive. Alternate translation: "But if I am to remain alive in my body" or "But if I continue to live" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The words "fruitful labor" here refer to work that has good results. Alternate translation: "that means I will be able to do work that produces good results" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-explicit]])