Mild to severe conditions can cause this Require proper examination and Tests

Please get few blood investigation n xray ,mri of lower back .. with these we can diagnose the cause of cramp n treat accordingly

Cramps in both legs during brisk walking can occur due to lumbar spine nerve compression related to your past slip disc, especially if symptoms appear after walking a certain distance and improve with rest other common causes include muscle fatigue, electrolyte imbalance, dehydration, or reduced blood flow to the legs because symptoms are activity-related and bilateral, conditions like lumbar canal narrowing (neurogenic claudication) should be considered.

Need some blood test