During the Six Nations, Farrell juggled his role at Saracens with his international duties and returned to his club after the Championship before rejecting the chance to join Stuart Lancaster's staff on a full-time basis in April.
But the 37-year-old former dual-code international quit Saracens in June and four weeks later signed a three-year deal with the RFU.
"At the back of your mind you have the chance to go on this journey with the national side, working with the cream of the country against the best in the world and at the start of something special," said Farrell.
"The hardest point was the emotional connection, at a cut-throat time in the season, with a club that has been good to you. Ultimately, you don't want to let people down.
"But I still think about Paris and the Welsh game and the atmosphere.
"It doesn't get much better in top-line sport than those occasions and we believe there are many more of those to come and that's what we want to be involved in."