Hi Ewan, welcome to MacRetro
I'm over in South Lanarkshire, so other side of the clyde. tbh I don't know you're neck of the woods that well.
Closer to you
Vague recollections of being told about trails up at some some loch or reservoir in Airdrie, glen something??
Cumbernauld, used to work there and I'm aware of trails at Palacerigg country park and there used to be group that did weekly rides.
Campsies - besides carron valley mentioned by hawmaw, there's plenty of trails up there unfortunately to date I've not explored. I guess studying a map for tracks or search tinternet for trails or walkers paths.
My local trails involve chatelherault country park, strathy park, barrons haugh reserve and the clyde walkway. the below is route recorded by a friend on one of our outings. There's plenty of variation, I get up to 25miles off road.
Also I can use the clyde walkway up to the trails round new lanark with minimal road involved. tour of lanarkshire sample route
, on Flickr
hope this gives you some ideas, let us all know if you find any interesting rides