United lost out on a 20th league title last year to rivals Manchester City on goal difference on the final day of the season, and the 71-year-old expected another tight battle.
Instead, the 2012-13 campaign has played out very differently, with the Red Devils storming to a 15-point lead over City. The United boss said he had also expected other teams to mount serious title challenges.
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"After last season, what we said was, 'Let's make sure we don't lose on goal difference', because that's what happened last year.
"Of course, our goal difference has improved dramatically, but I never thought for a minute we would be 15 points clear at this stage of the season."
The Premier League title is all but his, but Sir Alex still hopes to finish the season strongly, but admits that the current international break is cause for concern for the United boss.
He continued: "Every coach in the country is praying that their players come back fit – that is the first thing.
"They fact that they have played last Friday and they have played tonight – we have a game at 12.30pm on Saturday away from home at Sunderland – so once I know later this evening how many minutes each player has played, I will start to think about my team for Saturday.
"I have to make sure I have some freshness in the team because we have Saturday lunchtime and then on Monday lunchtime we have Chelsea [in the FA Cup quarter-final replay], so it is a big ask for our squad and something that we really need to concentrate on and make sure we need to pick the right teams."