I've noticed there are many posts about knee and leg pains etc, the app only mentions the brisk walk as a warm up and this is not sufficient at least not for me, after my second run with crippling lower leg pain I researched (googled) exercises for runners and haven't looked back as they say, I spend maybe 20 minutes before my brisk walk doing simple leg and back exercises / stretches and always end on the stairs with my feet half on a tread raising and lowering myself to warm my Achilles tendons which I do before racket sports too, too often we jump into action not properly warmed up I'm surprised C25k don't mention it perhaps they do and I haven't spotted it, I also use my foam roller on my legs, upper and lower, using body weight they are brilliant I'll try to add a photo
The leg press at the gym is good for my knee issues, talk it over with a trainer there who can diagnose if it's good for you too