Are you using any mods? A lot of mods aren't made to be used in the editors and so cause a bunch of issues if they aren't disabled. If you don't need any mods in the editor, the best thing is to use the -disableMods launch option.
Landscaping tools do not work well with the brush size set to "single" - increase it to show a number and it will behave much better.
You will also want to turn the strength down a lot to something like 0.1