I've found recently that when on longer runs, whether leisurely or not, I have been overheating and had to stop myself. This was a serious problem and extraordinarily frustrating. I don't know if anyone else is in this same boat as me but I found this company that makes shirts designed to keep athletes cool during training and competition. It's called ActiCool and I highly recommend it, as it has completely changed my training and I can work harder than ever before. Check it out at https://acticool.net/ if you want to know more, I highly recommend it.