my sis and her bf walked this trail last year and came back totally bewitched. they went in self organization, with a tent, a cooker and with a whole set of pre-prepared meals. initially they wanted to walk south-north but because of the prevalent northern wind decided to walk north-south - which proved to be a very good idea. they did no hard training beforehand, as far as I know. the walking was mostly ok underfoot (sometimes a bit stony, sometimes muddy, even an odd patch of snow or two), the landscape amazing, but the distances often seemed longer then on the map - probably because of the landscape changing so little. photos they brought back were magical. they stayed an extra day at the start in landmannalaugar to explore its surroundings.
hope this helps you a bit!