If you have your phone with you, try and get a photo of him. It sucks that you had to cut short your workout. It may be that what you experienced as “staring”, he thought of as “looking” - men can be oblivious to how they are perceived - but that’s no excuse.

I remember this one guy telling a bunch of us how he’d “helped” a woman late one night, by walking behind her on a deserted street, “to see that no harm came to her”. Boy did we lay into him. In the end he understood that a) he had actually stalked this poor woman and b) next time he should cross the road so she had one less thing to worry about. What a dipshit.