“Useful impact” is subjective depending on where and when you place the runs within your cycling schedule.
The run intensity (tempo) can have different affects based on running experience and overall fitness. I would say you will only know through experimentation. However, as mentioned, be mindful of where you place them within your weekly cycling schedule, and the time of day (ORDER) you do your runs and cycling to allow for adequate rest and recovery. The added runs, if done at proper tempo efforts, will increase your TSS. Whether they are useful or not depends on your goals and how your mind/body responds.