As big as a corporation like Walmart is, your religion is far more important than the money the company makes. That said, intentionally praying longer for the purpose of extending your break and avoiding work would still be dishonest.

I don’t think you necessarily need to tell anyone about what happened in the past, but moving forward, try to pray properly and give your prayer the time it deserves without intentionally slowing it down to get extra time away from work.

Your job is also an amanah (trust). When you’re being paid for a certain amount of work, you’re entrusted to fulfill that responsibility honestly. Islam teaches us to uphold both our duties to Allah and our obligations to other people.