That Fitness submenu presents the option to create a Fitness Profile as well as instructions to say “Alexa, start a workout” to start tracking.
In its current form, workout tracking with Echo Buds appears to be rather basic. There’s no support for heart rate monitoring, with the app instead only recording time, distance, pace, and steps. That data is then read back by Alexa after the workout, and can be viewed after the fact in the Alexa app itself.
I said this and it said “OK, starting your workout.” I walked around the office with the Echo Buds in my ears, and it appeared to have tracked my movement, telling me, “You’ve worked out for 1 minute and 37 seconds. You’ve logged 0.04 miles with an average pace of 44 minutes and 13 seconds per mile. You took 114 steps.”