The export will be the selected range. Therefore, if your tracks are on separate clips, click the clip before exporting. Then you will only export the exact length of that clip. It also only exports selected tracks.
Keep in mind where the track is routed. Sometimes, people bounce a track through a master bus with a bunch of fx on it. Then they export the new track. If the new track defaults to the master bus then the track will route through all those fx again. So, instead, route the track direct to your interface so that no additional fx are applied.