It's up to you - if you normally only include 1-2 walks per day when you are boarding dogs, then your overnight price includes 1-2 walks. If adding an additional walk or park time to your routine is a significant change to your routine, than it makes sense to adjust your rate so you get compensated for the extra effort, especially if it's something you're doing only for this one guest.
I usually do 1-2 walks per day, but I also vary activities depending on the needs of the dog, so for mellow dogs, just walks are great, but for higher energy dogs I can take them out with the bike jogger so they can run as fast and as far as they're comfortable, or arrange playdates with friends' dogs (I don't do dog parks generally with dogs I don't know well, and neither of my own dogs enjoy them very much), or go to a safe, enclosed area we can play fetch off leash. I also love to pose the dogs to get great pictures for the owners, so I work with a lot of my guest dogs on some basic posing and self control (like sitting and waiting on a park bench, etc), which helps wear them out mentally. But in nearly all cases, the extra activities benefit my dogs as well, so I wouldn't charge extra. In your case, where the owner is asking for a very specific routine, I'd be more likely to charge extra as it would be more likely to disrupt my day, and it would necessitate me leaving my own dogs/any other guest during the park time.