Most efficient endurance exercise for a 57 day time period?

I have 57 days to be able to swim a 500. I was able to swim 3 consecutive 500s with about 2 minute rests back in February on the swim team, but have done virtually no cardio since then. I am worried I will not be able to perform. I don't have access to a pool, so what are the most effective land endurance exercises for a 57 day time frame?