Foot and knee pain

[bunnings]

Hello l am a 42 year old overweight male of 135 kilos. Past 5 months l've been getting pain in the left hand side of my left foot below the ankle and the past month l've started getting a slight aching pain in my right knee. My doctor said  to get better shoes with strong support and cushioning and lose weight it will get better, the shoes has helped but the pain its still there when l walk and put weight on my foot and knee. l am OK walking short disatances , but it gets painful if l walk longer then 15 minutes . l know losing weight is the solution but its difficult as l cant do much walking but l am on a diet and slowly losing weight but it takes time. l am wondering if you have any advice on how to make it better, is losing weight the only way  that will cure the problem, could my weight have casued permanent damage and the pain will be there for life?