MUKUCHI NA OMOI WA KOI TO NARU

During middle school socially anxious Yuki experienced losing a running competition for the first time. When entering high school he met his rival again and he gets swept up by his rival in joining the running club in high school. The bubbly personality of his rival Kawai are slowly but surely also bringing changes to himself. Unsure of these changes Yuki tries to push Kawai away, but Kawai doesn't want to accept this. — Description from AniList community (anilist.co).