Well, I'm not exactly certain about distance (I'm sure others are!) without looking it up, but to walk from the main gate to the back of the park (let's say, Mean Streak) would take about 30 minutes if you're just strolling along. It sounds big, especially when you consider how many coasters the park has plus all the other attractions, but it is really laid out well and fairly compact. Just wear good walking shoes, as with any amusement park!
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/