In town, or walking to work, I walk just under 4mph (what's that, 6kph?) and so I used to assume a pace of 3mph (4.5kph??) when planning a mountain walk. In practise I'm nothing like that fast - for instance, the seven miles of the Snowdon Horseshoe took me 6.5 hours, and last week a 19-mile walk on the Pembrokeshire Coast Path took me nearly eight hours (though I was shocked to see I'd done over 3000 feet of ascent on what seemed a pretty flat walk). I stop to admire views far too frequently, and if there's a nice grassy patch overlooking a valley I'll invariably sit on it and have a fag and a snickers. This all adds up a lot. So I've found that, like a job, a walk will expand to take up as much time as is available.
When I started walking, I used to go full-pelt from the start and then wonder why I was struggling after the first hour. Learning to pace yourself takes time and as others have suggested, a group will walk at a much more sensible pace than an individual. But really, it doesn't matter how fast you're going, does it? As long as you're out and enjoying yourself, I see no reason to care. The right pace is whatever you're comfortable at.