Great job on the weight loss!
What is your time frame? What is the distance? How much walking have you been doing? Times per week? Duration?
I know this is contradictory to what Steven said but I would NOT recommend sprinting yet until you lose more weight. You are very correct when worry about your knees/lower body.
"And for crying out loud. Don't go into the pain cave. I can't stress this enough. Your Totem Animal won't be in there to help you. You'll be on your own. The Pain Cave is for cowards.
Pain is your companion, don't go hide from it."