Prep for Climb

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Hello All.
In March I am participating in the Fight for Air Climb in Michigan, which involves climbing 70 flights (1035 steps) of stairs. My gym does not have a stair climber and I am looking for ideas on how to prep. Being as this is Michigan, doing football stadium steps would be pretty out of the question since it's snowy, icy and freezing. Any ideas on things I could do in the gym without a stair stepper to help me prep for this? Thanks!