I've walked the Nijmegen march (4 daagse) 3 times in the past, it's really great so if you have the chance to go I can recommend it.
But do NOT go if you don't have a lot of training on marching on hard surfaces in flat terrain. Have at least 300-400km training before you go, start easy with 10km marching without any weight and then increase the distance and weight until you're able to march 45km in a day with 10Kg in your backpack.
It's possible to do the march without too much training, but it's a high risk that you'll get problems with your feet and back when you get older if you don't train properly before going there.
Marching on flat and hard surfaces is no way near walking in forest areas or forest trails so you'll need specific training before you go even if you're used to hiking a lot.
Here's the official Nijmegen 4 daagse site, you can find some info and tips there as well: http://www.4daagse.nl/index.php/en.html
And I've of course also done the Norwegian 18mile/30km march quite a few times (12-15 times), since I'm from Norway..