No. They don't, its all about figures. Think about it you're a business. You can either say the new 20$ DLC your making lasts only 5 hours if you do what the majority will do (Fast travel) OR you can say its up to 20 hours, but 'forget' to mention thats only if you walk every where and take your sweet time doing everything. Sorry to break it to you mate but no one cares about honesty. It's all about money, good numbers, sales, Argonian Skin Boots and Khajiit fur rugs.(With the Spiked [censored] attachment optional)
Even if you take things like Fast Travel into consideration, you're not going to be fast traveling everywhere. You still have to walk to locations, still have to delve through the Soul Cairn, and we don't even know if you can even fast travel in the Soul Cairn. Since it's an all new realm and through the information released in E3, it's just as big as Shivering Isles. That DLC took me ten hours to finish with fast travel and a modded character that could kill everything except the strongest Knights of Order with one hit.
For an average experienced character (Level 30 - Level 50), the DLC will last a bit. From a guy who played the demo, he stated that the Soul Cairn is similar to Sovngarde but much larger with castles, fortresses, and everything in between all dotting the landscape. Not to mention that he also stated that there's also ANOTHER area just as big to explore.
Not to mention you can have fun with the Vampire Lords or whatever the Dawnguard guys get (Don't say crossbows. I'm getting crossbows as a vampire. Just need to slaughter about 100 Dawnguard members I'm guessing).