In reply to Tony the Blade:
I'm not sure where you're based but the Pilgrim Challenge is down on the North Downs in February and could be just what you're looking for. It's two days, 33 miles each (so around 50km) mostly on trail, with some decent height gain about (1000m). You can just do one of the days if you like. I was able to get down there and back from London on the train too (they pick you up from the station if required).
It was my first ever post-marathon distance and I enjoyed it - a gentle but challenging step up. Here's the run data so you can get an idea of the profile.
http://app.strava.com/activities/18590940
It was well organised and friendly, although the year after I ran they had massive snowfall and freezing temperatures so that time of year it could be a bit of a lottery (cold, dry and clear when I ran it).