The Royals' relegation to the Championship was confirmed on 28 April when they drew 0-0 with QPR, who will also be competing in the second tier of English football next season.
Mariappa spent six seasons in the Championship with Watford and has therefore experienced first-hand the challenge that lies ahead for Nigel Adkins' side.
"The Championship can be a relentless league - it's all about recovering and preparing for the next game," he said.
"There are some very good sides in there and I feel like the quality is improving every year. Next season will be a very tough year, but we'll be ready."
Despite the threat of players leaving the Madejski Stadium in the wake of relegation, Mariappa is keen for the bulk of the squad to stay together.
"The more you play with people, the better relationships and understanding you have with them," he added.
"But there's lots of competition for places and I wouldn't say that anyone has a position nailed down.
"Obviously you want to keep your best players at the club, and we've got a lot of good players here.
"We want to keep everyone together so we can have a strong season next year and hopefully bounce back - that will be the aim."