Its very possible to go to Marsabit Town from Nairobi in a day, but that will depend on mode of transport.
Modes of Transport;
1. Airlift is hard to get, unless you work with an international NGO, or you can charter one.
2. Road transport can be achieved by;
- Buses at Eastleigh, they will take like forever to reach and you will be very tired.
- Matatus from Nairobi to Isiolo (via Meru or Nanyuki), then bus or Land Cruisers at Isiolo to Marsabit. The Land Cruisers (Not Public Transport) are tough to get, but are MUCH MUCH FASTER.
Fastest Road Route is the use of the A2 Road (i.e. Nairobi-Karatina-Nanyuki-Isiolo-Merille-Marsabit). Nairobi to Isiolo can be achieved in 3 hours. Isiolo to Merille can be max 2 hours. However, Merille-Laisamis-Logologo-Karare-Marsabit is HELLISH, and minimum can be 6 hours (yet its less than 150km because of the vehicles tearing corrugation).
Security is not a major issue.
Receive with simplicity everything that happens to you.” ― Rashi