Beginning at the South Rim, you have two options to descend into the canyon. The Bright Angel Trail (depicted here) offers a steady descent into the canyon and multiple sources of water along the way. 4.5 miles from the trailhead, Indian Garden provides year-round water with the next dependable source at Phantom Ranch across the Colorado River foot bridge.

Alternatively, some choose the South Kaibab Trail down to Phantom Ranch. Less visited and slightly shorter, the trail is much steeper and the same route utilized by a mule train that leaves around 5am every morning (make sure to start your run before they do, they aren't keen to allowing other trail users by in the dark). Some runners choose to go down South Kaibab and up Bright Angel. If you choose that option, check on the bus schedule to make sure you will have a ride back to your campsite or vehicle.

Organized groups are required to obtain a permit through the National Park. There is also an entry fee to get into the park, and most runners choose to camp there during their trip. The park's popularity makes it difficult to secure campsites during certain times of the year, so plan accordingly and reserve a spot far before your trip.

The traverse is best attempted in the fall and spring, when the weather is at its best. Seasonal water sources along the trail are turned off mid-November, so check with the park regarding the latest status. The pipeline that provides water along the trail is also outdated and experiences frequent shutdowns for maintenance. Always carry a water filter or other purification system just in case.

Though the entire run is loaded with views, the push up to the north rim is often the most memorable for trail runners. Narrow bench cuts along rock bluffs and several bridges line the way up. Even in November, it's possible for the bottom of the canyon to reach 70 degrees during the day, with deep snow blanketing the north rim.

Sources: https://ultrarunning.com/features/destinations/grand-canyon-basics-rim-to-rim-to-rim/