Times on offroad maras vs "normal"

Hi - Can anyone give a guess as to how much longer an offroad forest marathon would take compared with a "normal" road marathon?

 The Oslo forest marathon is coming up very shortly. This is run of XC ski trails, dirt roads, fire trails in forest, and the like. Up and down somewhat, but not gigantic hills. I wondered if anyone with experience of similar marathons could give a guess or their experience on how much longer such marathons take. I'm finding it tricky to estimate my time, and I like to program a Forerunner target time to run against. If, for example, I'm in form to run an ordinary mara in 4:00, what would you say: 4:20, 4:30?  Thanks for any tips or opinions.

That's an interesting idea - thanks Sheddy. There are results for the Oslo forest marathon here. There are results http://www.oslomaraton.no/ on a tab here.

In 2007 in my age group M50-55 the field was about the same size (40 odd). The fastest and slowest were 6% and 2% slower in the forest than the road. I must say though that the forest thing tends to attract fewer one-off runners and more nutters On that basis a 10% allowance seems to be a fair margin since I want to be conservative and complete it rather than try for a pb. (My best was 3:59 5 weeks previously in Prague.)

On the basis of FB's experience perhaps I should expect more like an hour extra. I have also had a cold virus in this 5-week period and have only just got over it. On the basis of my last sunday 15 mile hilly run yesterday I should actually expect about 4:45. Maybe 4:30 i.e. 12.5% extra would be challenge enough! It is dry at present but if it rains and gets muddy I could make an extra allowance for that

WA I suppose I could ask around on the start line - the only guy I have spoken to so far actually came over from the USA once and he called it undulating and pleasant scenery - there are just a couple of bits of technical single track. But I want to do it a bit quicker than he did

I suppose another alterntive is to run it on HR - I know what I can sustain from the Prague, and that would also take into account any lingering virus effects.

Thanks folks!

Follow-up: the event is finished, and I ran 4:25 on good gravel and earth trails of fire-road type (mostly) with a total climbing and descent of about 1500 meters. This compares with 3:59 I ran in Prague 5 weeks ago. So the result was that I ran about 11% slower off-road (if anyone is looking for a similar ballpark guideline).

However, it was only a week since the effects of a 2-week cold virus cleared up, and it's hard to say whether 5 weeks is enough for recovery from a mara where I tried for pb. Plus I ran on heartrate rather than a decided goal, so all in all I'd say 11% is close to a worst-case estimate, so long as the surface is reasonably OK and there is no extra speed loss due to mud etc.

Of course, the scenery was absolutely beautiful, the location is a cinch to get to from the airport, and I'd recommend it to anyone!