“Brittany Runs a Marathon,” as a film title, says it all. Brittany, ably and pungently played by Jillian Bell, is spurred by self-repugnance at her weight to commence jogging from scratch, and eventually to tackle the New York Marathon. It’s a light-hearted comedy/drama that shines in its dialogue and the running scenes, but sometimes falters at transition points. As a slow jogger myself, I experienced many “ah-hah” moments but the laughs are wry rather than heartfelt, and the ending fails to milk the film’s complex set of themes. An enjoyable evening’s entertainment with no memorable hook.