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Go to the event finder on RW and search your required area - anything with Hell, Extreme, Mayhem, Challenge or similar in the title are usually "something different"
Find your local off road club. You will normally find they run a winter league with other local clubs, so you will be able to do several events that are different from the norm over the coming months.
There's the Maldon Mud race around this time of year - it's across an estuary when the tide is out. I think it's a bit muddy, and rather cold, but not very long. Never done this one.
Or the Benfleet 15 - I think that's in January or Feb, some off-road, again a bit muddy. As in shoes left behind muddy.
Race the Train
Man vs Horse
North Downs 30k - Kent, June, mostly off road, rather hilly.
Transbaie in northern France - across the bay of the Somme and back, while the tide is out. That one is in early summer, depending on when the tides are right on a Sunday. More mud - smelly mud, too, suck-your-shoes-off stuff if you don't get your technique right. Good fun, that one!
Marseille-Cassis - October, point to point, about 20k, the first half is uphill, the second is downhill. Great views.
Gendarmes et Voleurs - near Limoges, May, a choice of three distances (5k, 10k, 32k), nearly all off road, very, very hilly.
Marvejols-Mende - half marathon, July, on the road but seriously, seriously hilly and can be seriously hot.
Sussex Obstacle Dash
I keep getting leaflets stuck to my car, wish I could remember them all.