Riding - to the warrior in the hall - is easy,
But very strenuous for one who sits on top,
Of a powerful horse over long miles.
In other words, for the warrior in the Hall (that is, one who has created a tsaheylu bond of being with her/his horse), riding is easy. For one who merely sits on top of the horse (that is, one who has no bond of being with her/his horse), riding is very strenuous, especially as the work continues over long periods of time.
This interpretation of Raidho is supported by the dream/runeworking shared here.