I have a circuit of cycle track that I walk daily with my dogs, around a mile stretch of the ex-railway line between Bedford & Sandy in fact which is now Sustains R51 and tarmac. I can easily fill a black bag every couple of weeks and the regular replenishment of waste is discarded by fellow walkers, cyclists and most recently the joggers are discarding their foil gel packs of energy juice (and single use water bottles) that prevents them competing their run otherwise! Then there are the general suppliers of beer cans and bottles, some of which are discarded by fishermen but not in all cases. There are four red dog-poo bins on the route but the bags still arrive in the bushes.
You are correct though in that if nobody cares and wants to pick anything they don't like up, then after a few years it would look appalling. Maybe it's just me and the human race is evolving more-and-more to love living in their own ****?