How much does a 15-minute walk after a meal really help blood sugar?

Measurably. Studies of short post-meal walking consistently show reduced glucose excursions compared with sitting, with effects visible on continuous glucose monitors within the first hour. The mechanism is that contracting muscle takes up glucose through a pathway that does not require insulin, which is why it works even in insulin resistance. Timing matters: starting within about thirty minutes of eating captures most of the benefit, and later walks do considerably less for that meal. Intensity does not need to be high — an easy stroll is sufficient, which is what makes it realistic to sustain.