After prostatectomy surgery I was told to do Kegel exercises to retrain bladder sphincter muscles. Does walking help to strengthen those muscles too?

Kegel yes. Hegel exercises are proper. Walking is good to retain body tone, but won,t do much for your bladder sphincter.
No. Walking has nothing to do with urinary control.Walking is good exercise for you though.
Probably not. Walking is good for you even if it does not strengthen your pelvic/sphincter muscles. Kegel exercises are specific pelvic-floor muscle exercises indicated if you have difficulty in controlling your urine right now. if you have good urine control you do not need to worry.