The Rhinos were bottom of the table after winning just two of their first 10 matches but have climbed to eighth thanks to four victories from their last six and now have their sights set on making it three in a row for the first time this year.
"It's been a challenging year," said Merrin, the former Australia Test forward who is in his first season in Super League.
"It's the first time I've been part of a competition that has relegation in it. It does give you goosebumps when you're down at the bottom, but I think it builds character too.
"It tests your strength as a team and the resilience of the group to stay away from it. Now we're looking up, not down.
"There's still a lot of room for improvement, but the last two months' of footy have been excellent. We've come up with some great performances.
"We haven't won three on the trot, so if we can tick this box, especially against the league leaders, it will be a great achievement."
Leeds' revival has been spearheaded by Merrin, who has missed only one of 25 league games and took over the captaincy when Kallum Watkins left for the NRL six weeks ago.
"It's been a great honour to captain such a proud club like Leeds Rhinos," he said. "It's been a privilege.
"My role doesn't change too much. It's a 'c' next to your name. I've just got to go about my job and lead from the front the best I possibly can.
"We've got some great young players here, it's just about leading them in the right direction and getting that passion back for the jersey."
The Rhinos could have back rower Stevie Ward making his comeback from a six-month injury lay-off, with coach Richard Agar likely to make a late decision on his fitness.
"We've missed him dearly this year," Merrin said. "It was sad to see him go down at the start of the year.
"He's worked really hard to put himself in a position to get back amongst the group."
St Helens are expected to be under-strength as they keep one eye on next week's Challenge Cup final, but Merrin is hoping to get the chance to topple a strong outfit.
"Our main focus is us anyway, but it would be good if they played their full-strength squad so we can really test ourselves," he said.
Saints have lost just three matches all year and wrapped up the League Leaders' Shield with five games in hand, but coach Justin Holbrook is expecting a close-fought tussle.
"Leeds over the last few weeks have been really, really impressive and the few new faces they have brought in have played a big part in that, in particular Robert Lui and Rhyse Martin," he said.
"They have been on fire of late, so they are going to be a real tough opposition on Thursday."
