Can you start PacePro (or change training plan) after starting an activity?

Today, I accidentally selected the wrong training - I used the one that Garmin had created for my marathon training plan ("recovery, 25 minutes") rather than the one I wanted to do ("weekly long run, 21km"). Unfortunately, I only realised after I started running and started to receive an endless stream of "pace too high! Pace too high!" messages.

Is there any way to switch training plan to a PacePro session (which I had created and synced to the watch) without stopping and deleting the activity? You can start a course after starting an activity, so why not a training plan?