Thanks for relating this. That is a seriously long walk. There was a period when a few of my good friends and I decided that we should trek across the US. Alas, it never came to fruition, but I always suspected that it would be brutally difficult. I think walking is interesting in that as long as your feet hold up, the rest of your body can put up with tons of it. Also, as evidenced by your post, you can't help but experience a bit of everything you pass when you are walking.
It really is all about the legs. For the two days following the first day, my feet and legs were aching really badly, and started to get better just as we ended up deciding to head back on the train. Working our legs up is something we really need to do if we plan to have more long walks. A trek across the States would be quite the undertaking! I imagine the distance would be similar to mine and Agata's eventual trans-Europe trip (just under 6000km).