12 miles every day your knees wont take it, I am a retired bricklayer because my knees are shot and waiting for knee replacements that just with using ladders and kneeling on wet surfaces. I take the pain in my knees , but I cannot take the depression pain has good, give me physical pain any day . You are addicted to walking if your doing 12 miles YOUR HOLE BODY WILL PAY IN THE END.