New manager Gerard Houllier has brought in Robert Duverne to be the club's new fitness coach from French club Arles-Avignon in order to work with the squad.
Duverne, 43, achieved global notoriety in the summer after his role in France's World Cup debacle in South Africa when he squared up to Manchester United's Patrice Evra.
Petrov believes fitness is on the up at the club and that no one is complaining at the stringent regime Houllier is putting his squad through in order to boost their success on the pitch.
"My next level of fitness is dying," joked the Bulgarian. "After the next couple of weeks I may do two marathons in a row!
"We have a new manager and he has pushed us really hard as he wants us to get fitter and to compete for the 90 minutes.
"We are making good progress and we will get fitter and fitter which is a good thing.
"Some days it's hard, it's physical, it's running other times it's football as well.
"We just need to accept it and keep working but we want to get fitter and get this club to the next level.
"We have a good fitness coach. He's been doing a really great job. He has his own programme and we are working on that direction."