I ........... for half an hour before I realised I was dangerously far away from the death
had been swimming
had swum
swam
was swimming
لان swim من الافعال التي تستغرق وقت لذا يوضع في الماضي التام المستمر كما انه عندك علامه من علاماته وهي for + مدة زمنية
والله اعلم