While I believe in Nintendo and its main developers are not ones to hype their own games, I would take this 70 hour quest claim with a grain of salt.
Sometimes developers exaggerate their games, making them out to be something they are not. The best recent example was Kingdom Hearts 2. Nomura and Square constantly boasted that the worlds would take more than 8 hours to complete, that the game featured had content that equaled 4 full length games and that the main quest would last around 80 hours. In reality, the worlds can be beaten in under an hour each and if you know what you are doing the game can be finished in LESS than 30 hours.
True, they could be counting the fact that you can level the characters up to level 100, level up the drive forms and the summons, acquire every synthesis item, acquire all the ultimate weapons, find every treasure and complete Jimminy's (sp?) journal. But even then, they kept boasting that the main quest would take a really long time to complete.
The way I see, a game's completion time is relative, it depends on the effort the player is willing to give it. For example, the original KH lasted around 30 hours, but I have more than 70 in my save because I spent it leveling up the characters, beating the extra bosses and getting the ingredients for the ultimate weapons. Another good example was Chibi Robo. In PGC's review, they stated that the game could be as long or as short as you want it to be. If you decide to just follow the main story, the game ends in a couple of hours. But if you decide to do EVERYTHING in the game, it will last you a good couple of weeks.
What I am getting at is don't be surprised if you find yourself going over or under the 70 hour promise in TP, and that if part of those 70 hours is spent doing sidequests and mini games.