I used to run lots of halfs and 10K's but never fancied a full marathon. I just didn't fancy all the road training.
But the 2 ultras I've done were mostly off road so thats where I try and do most of my long training runs (20-30m), which is far more enjoyable and scenic.
I've still not done, and don't intend doing, a 'proper' marathon!
On-line entries for the Highland Fling Race 2015 (53m from Milngavie to Tyndrum) open on Sunday 12th October, 21:00 - only the 850 places up for grabs!