Hi Zaini, have you ever heard about hormesis?Briefly, hormesis is a biological phenomenon whereby a beneficial effect (improved health, stress tolerance, growth or longevity) results from exposure to low doses of an agent that is otherwise toxic or lethal when given at higher doses.This concept could help you to understand the body's adaptive response to oxidative stress by exercise.
You also check this new review, you'll find out many interesting things about the link between physical activity and oxidative stress
Nikolaidis MG, Kyparos A, Spanou C, Paschalis V, Theodorou AA, Vrabas IS. Redox biology of exercise: an integrative and comparative consideration of some overlooked issues. J Exp Biol. 2012 May 15;215(Pt 10):1615-25. doi: 10.1242/jeb.067470.