"We went through 10km and I knew it was quite fast, and knowing the last two miles were going to be windy, I was thinking I must keep some energy," added the 26-year-old.
Farah, the first British winner since Gary Staines secured his third win in 1996, has played down suggestions his participation in Portsmouth might signal his intention to step up to longer distances next season.
Great Britain's Mo Farah talks to reporters after winning the Great South Run in Portsmouth after a thrilling sprint finish.
Ines Monteiro led the women's race from start to finish to win in a new Portuguese national best of 52 minutes and 32 seconds.
She came home 26 seconds ahead of compatriot Ana Dulce Felix, with Australian Benita Johnson, the 2004 winner, third in 54 minutes 41 seconds.
Pre-race favourite Catherine Ndereba of Kenya struggled and came home in fifth place, almost three minutes behind Monteiro.
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