A 0-0 draw away at bottom club Reading on Sunday condemned both teams to life in the second tier next season, their top-flight exits coming at a time when the division's elite are set to benefit from a bumper TV rights deal.
"There are some good teams in the Championship and the first thing I said to them just now, we have to come back pre-season, get super-fit because the Championship is Saturday, midweek, Saturday, midweek...," Redknapp told Sky Sports.
"If you are not fit enough to go the pace you won't survive. You won't get out of that league. Whichever players are here, they have to be worked on so hard and they have to be willing to get super fit to compete at that level next year."
QPR, who finished one place and one point above the relegation zone last season, have won just four of 35 league matches in this campaign, unable to recover from a dismal start where they failed to win in their first 16 games.
Mark Hughes was sacked in late November, leading to former Tottenham Hotspur manager Redknapp's appointment.
QPR won a league game at the 17th attempt in mid-December but despite an eye-catching victory at Chelsea on Jan. 2, and back to back wins in early March, Rangers fell woefully short.
Nicknamed "Harry Houdini" with a reputation for pulling off survival scraps, Redknapp was unable to conjure a lasting revival with four defeats and two draws in their last six games cementing their demise.
"There is a lot of work to be done here. A lot of work to get promoted.. Make no mistakes about that," Redknapp added.
"It's a big challenge, there are some massive clubs in the Championship. We have to work very very hard in the summer and during pre-season. It's a really good challenge but it's not going to be easy."
After flirting with relegation last year, QPR and Hughes splashed the cash in the close season, their splurge including the signings of Julio Cesar, Park Ji-sung, Esteban Granero and Stephane Mbia.
Redknapp was also given money in the January transfer window, buying striker Loic Remy from Olympique Marseille and defender Christopher Samba for a club record fee.
Instead of helping gel a struggling team, the wages paid to the high earners caused a dressing room split and affected team spirit, according to Redknapp.
"You are not always going to get on. It was well documented by some of the players at the club that there were certain groups that were here before that felt some of the new lads were getting the big money," he said.
"That was always an issue in their minds and that's a little bit how it's been."
Redknapp now plans to sit down with Malaysian entrepreneur and club chairman Tony Fernandes to start planning for next season.
"I'll speak to Tony. He's over with the other investors this week. Well have some meetings and discuss how we see things going forward. As I've said many times, they are good people."
Fernandes took to Twitter to say "sorry" to QPR fans.
"We owe it to you. Took three years to get Caterham (Formula 1 team) right. No quitting," said the Caterham F1 team owner.
"Planning starts tomorrow. Meeting (manager) Harry Redknapp at 11 to discuss squad. Going to be tough but we will come back stronger."