I suppose that very much depends upon the walking, KimE. If it is strenuous, then lack of calories could very much be a problem. I've certainly 'bonked' whilst on a cyclosportife event (Bealach na Ba), and it was not pleasant; not pleasant at all.
I know the Bealach na Ba and can understand why it is not suitable for a bonk - the hairpin bends are just too steep.