Because of the pain in my lower back, my legs felt heavy as lead and I was walking as if dragging them.
Literal
Lower-back [genitive-の] pain [from-から] legs [subject-が] lead [like-のように] heavily dragging-[manner-ように] was-walking.
A vivid physical description stacking two ように constructions: 鉛のように重く ('heavy like lead,' a simile) and 引きずるように歩く ('walking as if dragging'). The から after 腰の痛み marks the cause — 'from the back pain.' 鉛 (なまり, lead) is a classic simile target for heaviness in both Japanese and English. The progressive ~ていました puts it in past continuous narrative.