The 37-year-old originally joined the team on an interim basis during the Six Nations but eventually decided to accept a permanent position under the leadership of Stuart Lancaster to end his time at Saracens.
Farrell said: "We had a great time when I was doing the interim job.
"I wanted to be involved [with England] but the dilemma was so great.
"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.
"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."
Lancaster revealed earlier this week that he has yet to have decided on who will captain the team during the autumn internationals.