The impressive 4-2 victory over Derby County at Deepdale on Saturday lifted the Lillywhites to 11th place in the first division table but Brown feels a play-off place might be just too much to ask.
"The earlier periods have cost us. If I said we were going to get to the play-offs, I'll be accused of being a nutter," he said.
"I think it's a bit too unrealistic. I don't want to be a killjoy, but I have to be sensible. We have to be honest and say we have left it a bit too late.
"It should have been a more comfortable win against Derby, a clean sheet. Carelessness at the back cost us a couple of goals.
"I was satisfied with the first hour. It was a bit of an anti-climax at the end, after doing so well."
Derby Match Report