Tom - I suspect that the main group of people who run their best marathon in terms of pacing are those who run only one marathon! Unless you're a very experienced runner I suggest you are conservative about your first marathon. 26.2 miles is a very long way. Most people do far better in later marathons.
If it can be fitted into your schedule I suggest you run a 20-mile race at slightly slower than target marathon pace say six weeks before your target marathon. That should give you an idea.
In terms of effort I usually find a marathon 4 times as hard as a half marathon, and slightly less than twice as hard as a 20-mile race.
Muddyfunster - Always good to hear how people started running and when. I'm 62 and started at 55, though thought I was just doing personal training with my wife and a friend until they dropped out leaving me and I found I really liked running. I'd hated it at school, though I did some running for a year in my 50s on a treadmill in a gym.
I was very keen and did loads of races 2-3 years ago, but racing is too injury-prone, so I don't race much now, just run a lot, not really following the plan because lactate threshold and intervals are too intense. Really enjoy the forum, though!
I run partly in Central London, partly near Tunbridge Wells. As you can see from the Strava group, even Central London seems hilly compared to some parts!
I'm hoping to run Bramley 20 this Sunday, maybe in 3 hours.
I wonder if my targets could be added to the list? I plan to run Brighton (17 April 2016) - target time 3H45 and London (24 April 2016) target time 4H30. I'm running a "sort-of" P&D up to 55 miles, with extra miles but less hard running.
Not very sensible, perhaps, to run two marathons 7 days apart, but I don't like to miss London if I happen to get an entry. I think Brighton is a better race for a good time.