In reply to jadm2: I was dragged out walking in the local Peak District and by 4 could manage 13 miles.
As Carolyn says flat walks are boring. Lots of scrambling makes things fun, and generally the rougher the terrain the better.
Kids use up loads of energy so yes sweets are not only a good bribe but also help to keep energy levels high. promises of ice-creams also help, but they will never let you forget the long hot slog up a steep hill only to find the shop was closed.
Give them a copy of the map with the planned walk highlighted and let them do the map reading (expect take over when they get bored).
I used to love walking over the moors on a compass bearing (being small and light I never sank in as badly as my parents), Dad set the bearing and followed behind to make sure we didn't get too lost.
Natural history, geology and archaeology all break up a walk into more manageable (fun) sections and give lots of opportunities for rests in between the walking.
Another thing I loved was the fact that it didn't matter how dirty I got. I could have my clothes caked in mud and the only consequence was being sent to have a bath when I got home