The X Factor host, who is joined by eight other celebrities on the Kenyan Comic Relief challenge, accepted that the adventure would be "tough" but rewarding.
He said: "We're all ready to go now. We've had a practise walk to try and get used to the heat, but it's going to be very tough - we all know that.
"The snake and spider briefing was interesting to say the least! It's safe to say that we will be watching where we tread."
Also participating in the 100km journey across the Kaisut Desert are Olly Murs, Strictly champ Kara Tointon, Radio 1 DJ Scott Mills and comedian Ronni Ancona.
Completing the lineup are presenters Lorraine Kelly and Nadia Sawalha, R&B star Craig David and broadcaster Peter White.