Runners are very active ducks, and robust healthwise, rarely suffering from illness. So if she appears lethargic and underweight there may well be something wrong.
Has she possibly picked up some sort of digestive infection from the ground or water? If she is a bit wobbly and thin, with watery, greenish droppings then that could indicate a gut infection of some sort, worming would be good this time of year as well.
Runners are naturally light physically, do you have any others to compare her weight too?
How old is she? Is this her first laying season? Swollen belly which feels hard could be eggbound (as has been suggested) or squashy maybe an internal layer. What is her diet like? This time of year runner ducks should be on layers pellets for the calcium as they are prolific layers.
Just some thoughts, however, only a vet would be abe to diagnose, do you have a local poultry friendly vet nearby? Good luck with her and please keep us posted!