Guernsey Walker
It can be seen that my "Round the Island" walks are divided into two sub-sections, "The East and South Coast Cliffs", and "The Flat West and Northeast Coasts". As this suggests, these have different characters.
Although I've called this section "Round the Island walks", it is not suggested that anyone tries to circumnavigate Guernsey in a single day! Though in fact, one of the local IT companies organises an annual sponsored "round the island" walk, which starts soon after the small hours of the morning, and in which many entrants go on until the evening.
You and I are (I hope) walking for pleasure - although if you want to tackle the annual "marathon" (which many islanders do) I'm sure the organisers would be delighted. It's now too late for 2011 though. (Information at http://www.itexwalk.gg). (Incidentally there is also a similar annual event in Jersey).
If you do want to do a pleasurably long walk, then St Peter Port to Pleinmont along the cliffs is a practical proposition in a normal day (I would expect to take seven hours or so). That's provided you can make transport arrangements at both ends, or you may find it convenient to use the Guernsey Bus service for one or both ends of the route (check out the bus timetable beforehand though).
I've done this cliff walk (once) in both directions, and for me Town to Pleinmont is the best way, partly because you get the morning sunrise on the east coast, then at the end of the day the sunset over Pleinmont. Also though because the approach to Pleinmont tends to be flatter. Going the other way, you leave more of the ups and downs until towards the end!
However, for the more normal among us (!) I have divided the coastal walk into short sections, with a return route to make a circular walk of each of them. Feel free to depart from my sections of course, either shortening the sectors (where possible) or putting one or more together to make a longer walk.