I allowed myself a walk at every water station - mainly because I found it difficult to run and drink/eat at the same time - although it was less than a minute each time so I'm not sure if it helped in any significant way. In fact, on the last couple of drink stations it was really hard to get going again after the brief walk.
One thing I was aware of was the feeling of everybody going faster than me at the start (even though my starting position was fairly near the back). I resisted the urge to go with the flow up and just plodded along at my own pace. I'm convinced that I wouldn't have completed the course if I'd gone any faster at the outset because my legs were screaming at me to stop running for the last 4 or 5 miles.
I'd be wary of upping your pace at the 21k mark. You will probably be feeling great at that stage - I think you'd be best just getting to the 30 - 35k mark with as much in reserve as possible.