If it seems like a crazy idea, then you should probably do it. In fact, scratch that, you definitely should do it. This is what Ziplining at the forest sounded like. The Forest is this wonderful place located in Kereita slightly past Limuru town. Flying Fox (another of its names) is about 23km from Nairobi and should take you at least an hour to get there- on a good day. One could use public means up to Limuru town, then get transport to Kereita from there – in case one is not driving.
Directions can be found here http://www.theforest.co.ke/map-and-directions/