Hi Duane:
I climbed Kili in 2001 and it was an amazing trip. The hard day you mentioned is summit day. You have to get up several hours before sunrise to start your climb, and reach the summit right around daylight. The climb can be slow going and can be quite exhausting. The hardest part of summit day is not the climb up though, but the down climb. The guides really want to get you off the mountain as quickly as possible. You summit, return to camp 4, pack up everything, and then have a VERY LONG hike off the mountain. Its hard to estimate, but my guess is that its 18 - 20 miles of serious hiking within a 24 hour period. While I was prepared to reach the summit, I was not prepared (or fully informed) of the distance of the out hike at all.
The out hike does not take you all the way off the mountain. There is a final camp and a 6 hour hike the next day to complete your trip. Its a nice hike through the rain forest, but nothing compared to the day before.
Overall, traveling to Africa, climbing Kili, and then getting to go on safari was a FANTASTIC experience and you will have the best time. Please make sure to keep us posted on your preparations and post trip log. We look forward to hearing more from you sir!