Translation:I could have walked.
"Yürüyebilmek" is "being able to walk". Removing the infinitive -mek, and adding -di to it sends it into past tense. So the correct translation for "I was able to walk is" "Yürüyebildim". Note that you would only use "yürüyebildim" if you actually did the walking.
Yürüyebilirdim is an odd construction. If I'm not mistaken, it uses "Yürüyebilir" as the verbal adjective "Can-walk". So its very literal meaning is "I was in good condition to walk = I was able to walk". BUT This form is never used to convey that meaning only. Rather, it is used in statements that involve regret or second-guessing, like "Yürüyebilirdim, ama taksiye bindim [I could have walked but I took a taxi]."