Tolerance To Pain, one of the five aspects of mental toughness, is something you can't avoid. Your legs will feel like they're on fire. Who's going to volunteer for that..?
You have to reframe the whole idea of pain. Here's some ideas:
"The more pain I can take the stronger my body will be."
"The more pain I can take the better I'll feel after having pushed myself."
"The more pain I can take - that also goes for my opponent. And clearly I can take more."
All pain is temporary. As Andy Roddick once put it, "I'll talk to my body about it tomorrow."