The four-time Olympic champion will sign off from track racing after August's World Championships in London before switching to the marathon.
But the 34-year-old Briton, who laid down a marker with an emphatic 5 000m victory at the Prefontaine Classic on Saturday, said he is unlikely to be marathon racing at the 2020 Olympics.
Asked if he expected to run in Tokyo, Farah told AFP: "I don't think I will be."
"It's too far. I miss my family. My kids are growing up fast. I'm away six months of the year training and it makes me sad not to see them growing up."
