To be fair, although it's partly expensive because it's being organised by Great Run, it's also going to be expensive to close city centre roads - a lot of the other Scottish marathons are quite rural and low-key, so they're not as expensive to put on.
Yes I know, but I think in the context of the OP's question she just means all off-road running in the countryside, which is how people seem to be answering it. I quite often follow long-distance walk routes when running on my own off-road, so as not to get lost, which don't always involve proper trails, usually more of a mixture of paths of varying quality and just cutting across fields and hoping you find the stile at the other side, and it depends what the weather's been like too (as to whether there is anything you might describe as a 'trail').
Haha - you lot are making trail running sound like going up Everest!.
I've always run off-road if that seems like a sensible place to do it (e.g. when visiting my parents, who live in a small market town accessed by busy main roads, the only places to go are across the fields & through the woods). And where I live now in the countryside I often run on a mix of roads and trails depending on where I'm trying to get to. So it hadn't really occurred to me that off-road is more difficult. But last week I took my friend 'hill running' with a group of hardcore nutters ('if you don't see a path, that'll be which way we're going') and was merrily skipping along when I realised that she had never done hill running before and was actually finding it quite hard. Oops. I think it was the uneven ground rather than the gradients.