I've travelled long distances, but I've always had my backpack and used transport, and can get by that way fairly easily in general.

I've been reading about long-distance walkers - people who walk across America in 87 days, for example. I'm assuming they can't carry a 15kg pack on their backs this whole way, or food and drink - do they usually have support?

Put it this way, if I (or you) were to do it, what would one need to achieve a long-distance walk (aside from being fit and slightly mad :)), and how would you ensure you had enough supplies?