Absolutely agree with the comments by Real Ale Man, Stolly and Crowhill.
The stiles on the run-in can also be difficult when your calves are knotting with cramp. Even worse, I struggled to get up the pavilion steps to my cuppa at the end...
As for time, see how you go, especially if most of your experience is on road.
Why not aim for 4 and half hours? You can always speed up in the later stages if you've still got spring in your legs
The surest way of having a miserable time is to go off too quickly and pay for it later. One danger (especially for a road runner) is to take the long flat stretch to Ribblehead as if doing a 10 miler. Ten minutes gained on this section can cost you half an hour later.