The former Royal Bank of Scotland director is overseeing a revamp of the company following the departure of chief executive Andrew Moss after a shareholder rebellion over his pay package and the company's stock market performance.
In May he took day-to-day control of Britain's second biggest insurer, which has large operations in Perth and Glasgow, and is pursuing plans to sell 16 underperforming businesses.
Mr McFarlane said: "On the one hand we are blessed with a terrific brand and really professional front-line staff who have the customer's interest fully at heart.
"On the other hand, culturally the organisation has been more used to collective decision-making and has moved more slowly as a result."
He said this had "inhibited progress" on reform but that the pace of change is accelerating.
Aviva said the sale of the US operation would take place "reasonably soon" although it is likely to be at a discount to its £2.4 billion value on Aviva's books.
It acquired the business for £2bn in 2006.
Another eight businesses will be sold next year.
Shares closed at 330.3p, up 1.8p or 0.55% on the day.
Capital freed through disposals could bolster the insurer's reserves and protect against a further deterioration in the troubled eurozone.
Aviva holds eurozone sovereign debt to generate income for customers in the region, and its exposure has worried some investors.
UK sales fell slightly to £8bn in the first nine months of 2012.
Its general insurance operating ratio, a measure of profitability, remained flat at 97%.
Aviva said trading conditions for its life insurance and pensions business in continental Europe remained subdued.
Sales in Spain fell to £934m for the first nine months of 2012 against £1.4bn in the same period of 2011.
In Italy it was a similar picture as sales fell from £2.5bn to £1.6bn.
Richard Hunter, head of equities at Hargreaves Lansdown Stockbrokers, said: "The twin targets of building financial strength and trimming non-core businesses are both firmly on track."
Aviva employs 1500 people in Perth to handle all the group's household claims, and housing its top-end underwriting expertise on commercial insurance.
Another 900 at Bishopbriggs near Glasgow run call centre-based sales and service for various brands and manage bodily injury claims.