Knee pain and need to workout for weight loss

Our user asked: "Hi, I've been dealing with some annoying pain in both knees for the past two weeks. I've had pain from time to time--which I usually feel when doing lunges during my workouts, but the last two weeks have been severe. I know the pain was aggravated by a 7-mile running race I did two weeks ago, which prompted me to have my knees checked by the doctor. The initial diagnosis was for "Runner's Knee". The pain centralized in the front for the most part. They did X-rays and told me I have "minimal arthritis", but didn't recommend I do anything else at this point. The pain still comes and goes..climbing and descending stairs mostly, and standing up from a chair after sitting for a while. I also hear a clicking or "crunching" noise in both knees when I extend my legs. I'm VERY frustrated because, before the 7-mile race, I was just starting to get active again and eating right, working on losing the 50 lbs I gained this year from an office job where I sit most of the day. The upsetting thing is that just 2 years ago i was in the best shape of my life..i did Crossfit everyday, yoga, I was a runner and I even coached a bootcamp. I was strong and was at my ideal weight. PLEASE tell me what to expect and how to proceed?? Is it alright for me to go back to how I used to work out (high-intensity strength and cross-training), despite my new knee condition?? Will I ever be able to run again? My doctor suggested swimming, but I heard it's not great to lose weight--i need to lose 50 lbs! I really hope someone replies to this. Thanks so much for your time and thoughts!"