I recently completed this 18 mile hike and it was breathtaking I reccomend any one whois in good shape and loves the outdoors to try this amazing hike. I started the hike at 8:40 in the morning and finished around 4:15. If you want to do this hike go to the Shuttle Station by the Mammoth Mountain ans take the shuttle to Agnew Meadows. I took the high trail on the way there and the river trail on the return journey. Some people say that the high trail tobe boring but I strongly disagree as the views along this part are breathtaking. While the high trail is more expansive the river trail seems more intimate. Along this 18 mile hike I crossed nearly 100 streams and creeks, saw manydifferent habitats and plants, saw beavers, dear, and many birds. Thousand Island Lake was amazing with a snowy mountain in the back and crystal clear water. On this journey you should bring plenty of water and snacks, and a light jacket as sometimes the wind can get chilly. The altitude did not really affect me even though im from San Diego and not accustomed to the high thin air. If you have time for a day hike in Mammoth then take this trail.