i have a good bit of experience in fighting with against viking fighters and so on. many people ignore aspects of there fighting. i would just address there were some elite viking warrior groups who trained from birth so would have been in good physical condition. also not all were farmers etc in there spare time many were traders and made up about 5%of the population who were employed to raid and fight.
diet wise look at the york finds the diets apparently were very good but there was huge amounts of ring worm etc to make things akward. but like soldiers of any day viking ones would have had physical attributes even small that would set them apart
