A 25-20 Championship final second leg victory at Sixways on Wednesday night completed a 46-32 success for the Warriors and confirmed Leeds Carnegie's relegation from the Premiership.
Leeds, whose Premiership campaign ended with defeat against Heineken Cup finalists Northampton 11 days ago, would have stayed up had Pirates prevailed.
But Worcester, the only Championship club that meet minimum Premiership entry criteria, were in no mood to blow their big chance as a club record capacity 12,000 crowd roared them home.
Hill said: "At 25-6 ahead, I was a little bit relaxed, and I think some of the players were too. But the Pirates came back - they are a good side.
"Good luck to Leeds coming down into this league. There are some tough teams, and it is very hard to get out of it. We are relieved we have done, but it is a battle every single week.
"We fell asleep a little bit, and the Pirates have some real talent. There were a few scary moments towards the end.
"It was a scary game for us. The Pirates had nothing to lose, they could come here and throw the ball around."
Wings Marcel Garvey and Miles Benjamin claimed touchdowns early in each half, while Warriors' former Leicester and England number 10 Andy Goode crucially kept the scoreboard ticking over through a breakaway try, a drop-goal, penalty and two conversions.
Full-back Rob Cook kicked two penalties for the Pirates and added conversions to centre Drew Locke's score and a late penalty try.
But an uphill struggle they faced before kick-off quickly assumed mountainous proportions as Goode's 15-point haul meant Worcester remained in control throughout to take the Championship title.
Hill added: "We've started thinking about next season. The Premiership is an uncompromising league, and we have got some work to do in the summer.
"But the confidence we will get from winning constantly this season is important. We need to take it and bottle it up.
"We will go in on a high, as the likes of Northampton and Harlequins have done in recent years. We will feel good about ourselves, but the hard work will start on June 20.
"Overall, I think we were the best team in the league, but it's still scary. You can slip up - there are so many potential pitfalls.
"It's not an easy league to get out of, and it is getting stronger every year.
"We didn't want to be in this league next year. The longer you are in it, the harder it is to get out."
Wednesday night's clash was Worcester's 31st game of a marathon Championship campaign, but they now return to the Premiership for a second stint among the elite, swapping places with Leeds.
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