I believe the legend that is Samuri still holds the 3 peaks fixed record.
I do do it offroad in fits and starts when I get round to it.
Originally a couple of INbreds (EBB - front disc brake only and Track End both V brakes which were more effective than the disc in use)
Then my Ferrous for a short while with both V brakes (SSUK 2006 champ ;0)
Now a Cotic Simple with both brakes.
If your doing steep stuff having both brakes helps enourmousley with being able to save your legs or feather the braking properly as locking slowing with the legs is a bit binary. Really struggled in mud and steep with a front brake only as if the front end let go you were left with no control and lots of dabbing and one legged pedalling and swearing a lot. The only trails I avoid doing are deep pedal cathching ruts and long long fireroad descents as it get's a bit dull on 32:16.
I also found front sus forks helped with the ability to plough through objects more efficiently :0)
Current fixed set up (but it's a SS at the moment)
Stupified Simple by tomlevell, on Flickr