I have to agree with Marrowshard on not responding at all when they balk/try to flee. Castle use to do the same thing at traffic noises and other barking dogs. I completely ignored him, no eye contact or verbal response and continued on at whatever pace we were already walking at. After about 3 weeks of daily walks, he now pays no attention to your run-of-the-mill traffic noise and he no longer tries to flee from a barking dog, rather he usually tries to greet (which in another problem in some situations).
As far as the walking goes, I wouldn’t go on a marathon or a high paced road march with a mastiff, but absolutely walk them. If you walk your dog properly it is great bonding time. I never let Castle lead or decide where we're walking. I subscribe to Ceaser's pack leader mentality. My fiancé is more of a play buddy, "let's have fun" mentality, and their certainly is a difference in how well he listens and behaves for us especially if he is off leash walking. We both think that he listens better for me. This past week has been rainy and Castle only got walked a few times and there was a HUGE difference in his behavior. He was unruly and very stubborn and inattentive. The nice thing though was that he returned to his normal self after resuming his regular walking schedule.
Castle usually gets 2 walks a day anywhere between 20 to 60 minutes depending on the weather, time of day, and our work schedules. If it isn't too hot shorter walks are usually at a faster pace. So while we don't encourage high-impact activity like wrestling, jumping up or off of things, walking in my opinion is a necessity.