I did the best one day trek in New Zealand called the Tongariro Crossing. It is a 17 km trek that takes about 4-6 hours depending on your speed and weather conditions. The weather report said that it was not going to rain past 7.30 am. It rained the whole day! I threw away my shoes after the hike! You are supposed to be able to see the most beautiful landscape on a clear day. Including Mt Doom from the Lord of the Rings. I couldn't see anything because of the weather! It was still a lot of fun.