The Dons looked to be heading for a fourth straight defeat in all competitions after Jake Hyde headed the home side in front after 64 minutes.
Luke Moore wasted a great chance to level with 15 minutes to go but Long's strike from outside the box deflected past Bees goalkeeper Graham Stack in the 84th minute to earn AFC Wimbledon a 1-1 draw.
Ardley's side remain in the relegation battle, just a point above the drop zone and only three clear of bottom side Barnet, and he is desperate to stay outside of the bottom two over Christmas.
"I'm very relieved," he said. "I don't think we coped with them in the first half, they played a diamond and we didn't quite get to grips with it so that was the basis of the half-time team talk.
"In the second half, we reorganised and I didn't think there was too many threats. Then we gave away a set piece. We've worked tirelessly on them, and we're going to have to go back to the drawing board.
"We showed great spirit, there were moments of quality, not much as it was a relegation fight, but great will power got us back into it and it's a great result for us.
"For the lads' mentality I think it's important we're out the relegation zone at Christmas but we know what we've got to do, we've got targets, we discussed them during the week. This was a big game but it's not going to decide our season"