We have hiked the Grand Canyon twice. Both times we hiked from the south rim, down on the South Kaibab, spent the night at Phantom Ranch and then hiked out the Bright Angel trail the next day.
Hiking to the Colorado river from the north rim is more problematic because the season that the north rim is open is so limited. By the time the north rim is open, the seven miles at the bottom of the canyon is going to be very hot. The north rim trail to the river is app 14 miles total; seven miles down and seven miles across the bottom of the canyon.
Hiking at that elevation is entirely different than flat country hiking. On the Bright angel trail, you will be hiking a vertical mile. Each of the last three miles has a 1000 ft elevation gain per mile. You are working the hardest at the end of the hike and the air is getting thinner as you go up.
If you haven't already done so, check out the National Park website at www.nps.gov/grca. Very good information there.