What kind of exercise? A modern smartphone is pretty light and easy to carry in all but the most extreme forms of activity. One of my friends takes her iPhone surfing.
Running? I wear loose-fitting athletic shorts; a phone swinging around in my pocket is really annoying. I'd have to get an armband.
Arm bands are cheap, much less than a smart watch.
I've had a very hard time finding an armband that fits around my arm, and fits an iPhone 6. They seem to all be designed for women.
Well you've found a new startup business opportunity: smartphone armbands for people with big arms. :-)