Campbell, 37, announced his retirement yesterday.
Campbell said: "It's hard to let go when you have been a top sportsman for a long, long time.
"To say that I am not going to play professional football again is difficult.
"I had a couple of offers to carry on - one from a Premier League team, and another from a team who have just been promoted to the Premier League.
"But I had a foot injury and by the time I'd recovered people had already moved on."