Having not played a game for 21 days, Hills' side prepared for the play-off with in-house friendly matches and high intensity training sessions to keep them match fit.
However, it did give them time to get over the disappointment of their final WSL 2 defeat by Birmingham City, which saw them drop from first to third in the table in the space of 90 minutes.
Very few had tipped Charlton for promotion contention at the start of the season, so they exceeded expectations by keeping up with the pack, but to miss out having led for so long would have knocked them.
It was "another bite at the cherry" in the play-off, manager Hills had said earlier in the week, and Charlton looked like the side with a spring in their step in a dominating first half.
They started brightly and the majority of their threat came down the right-hand side through Karin Muya, who was a standout performer before she was substituted.
But it was the ever-reliable Whitehouse, the Charlton goalkeeper who kept eight clean sheets this season, who kept them in the game on several occasions.
Her stunning save to deny O'Brien meant the teams were still tied 0-0 at half-time, before later reacting well to block Neville's header at the back post and ensured the game went into extra-time.
It was only fitting that she proved to be their hero yet again with four remarkable saves in the shootout, meaning she was swarmed by team-mates at the final whistle.
This is a remarkable achievement for a club who have competed with many others in the WSL 2 on much larger budgets and backing from their higher-ranked men's teams.
It is also a deserved return to the WSL for former Spurs manager Hills, who guided Tottenham to the top-flight for the first time in their history back in 2019.
"I'm excited, nervous and apprehensive because I've done it before. It's tough," said Hills afterwards.
"We have the best league in the world with some of the best players in the world. We have to be realistic and honest.
"We are a very progressive club. I was here as a player when we were a top team so that is my ambition again. Football doesn't stand still so we have to stay with it."