Christy Turlington Burns, Apple's Jay Blahnik to talk Apple Watch & running at Boston Apple Store

The event begins at 3 p.m. local time, and people must RSVP to attend. Moderating the discussion will be Jay Blahnik, Apple's director of Fitness and Health Technologies.

Other guests will include ultramarathon runner Scott Jurek, and Heather Armstrong, a professional blogger. All three are running to support Burns' organization Every Mother Counts, which works on improving maternal healthcare programs worldwide.

Burns has been a key marketing tool for the Apple Watch, appearing on stage at Apple events as well as in a pre-release diary that highlighted the product's features. She sat with Blahnik at the Regent Street Apple Store in London last year. In exchange for her help, Apple has allowed Burns to draw more attention to Every Mother Counts.

Although it's not heavily fitness-oriented, the Apple Watch does track steps, heartrate, and standing time, like many mid-range fitness trackers.

It's not clear what a next-generation model might do to improve tracking, but the device is expected to have better internal hardware, while remaining mostly the same in external design.