Fixed amount calculations

src/models/entities/Runner.ts

@@ -54,7 +54,7 @@ export class Runner extends Participant {
    * Returns the total distance ran by this runner.
   */
   @IsInt()
-  public get distance(): number {
-    return 0;
+  public async distance(): Promise<number> {
+    return await (await this.getValidScans()).reduce((sum, current) => sum + current.distance, 0);
   }
 }