I really didn't. I did some 5 mile walks in the spring, and one 10 mile walk in July. But I'm pretty limited on time, so whenever I did have a 2-3 hour block to do something, the bike won every time! If you are in good enough shape to do centuries, then you'll have the legs to walk. The key for me to keeping my feet happy was GOOD socks and shoes...no skimping, and start testing the different types early. I swear by Thorolos, but others prefer thinner socks like Wrightsocks. I would also slather my feet with bodyglide to cut the blister causing friction.

For me, like with any "event" walking or riding, a big part of it was mental. Once I paid my registration fee, I was committed 100%. I never let myself have doubts about making it, I just did it.