In reply to awidgery:
Mountain club of Kenya website and Lonely Planet's thorn tree forum have more information.
Obviously read guidebooks and maps from Cordee / Lonely Planet etc.
Considering your experience, you will probably conclude it is simpler to be self-guided.
If you are coming from sea level, you may take a bit longer to acclimatise than people who have been living in Nairobi so allow time and enjoy the walk.