If i walk for a period of time my knee aches, what to do?

Depends what hurts: Need more information. Your knee can ache from having worn cartilage surfaces called osteoarthritis. It can happen from torn meniscus which is your bumper in your knee protecting the cartilage surfaces. It can be from tendonitis or bursitis. I would get an exam from an orthopaedic surgeon and find out the cause and get recommendations.

See orthopedic doc: Knee pain after walking is cause to see an orthopedic doctor. The cause are many and someone needs to sort it out to offer a solution.