Need to make my legs stronger. I normally average around 15,000 steps a day. Today I hit 35,000, and my legs are sore, they always are. Any tips on making them more acceptable to an active job? (20,000 steps on an average day)

Need to make my legs stronger. I normally average around 15,000 steps a day. Today I hit 35,000, and my legs are sore, they always are. Any tips on making them more acceptable to an active job? (20,000 steps on an average day)