Background: I have started running regularly with a partner who has a smaller stride than I do. Because there is only so much time in the day, my partner-running workouts have replaced my normal solo-running workout. The partner-running workouts are something I would like to continue with for multiple reasons beyond fitness goals.
Problem: My partner-running workouts last the same duration as my normal running workout (~50 mins), but because I am matching my running partner's pace, I am not getting my heart rate up into my target training zones as I would when running on my own.
Question: I have heard tips of exaggerating motion (e.g. high knees) to increase the effort / heart rate, but I do not want to regularly practice poor form (practice makes permanent) since this is something I have started doing every other day. Is there anything else I can do to improve my own cardio benefit while running with a partner that does not require poor form?