The Governor's Promenade starts on the grassy hill on the far side of the Citadel at a viewing point on Ave du Cap Diamant. It's a boardwalk that connects to Dufferin Terrace and allows visitors an optional route to return to Chateau Frontenac when visiting the Citadel and the Museum of Plains of Abraham. This boardwalk has many steps because of the differences in elevation so I recommend starting this walk from the Citadel area in order to walk down the steps to reach the Dufferin Terrace section. Walking this route may take about 20 minutes.
If you start this walk from the Dufferin Terrace, the section called the Governor's Promenade begins just beyond the sloping Dufferin Slides, just past the foot of Rue St-Denis. This walkway between the Dufferin Slides and the Citadel is narrower than the broad Dufferin Terrace boardwalk.
If you are just looking for a spot to see panoramic views of the river and beyond without tree stops obscuring the view, I suggest walking up to La Terrasse St-Denis close to the Dufferin Slides. It's the hilly grassy area just off the foot of Rue St-Denis, accessible by a few stairs from Dufferin Terrace. It takes less time and effort than walking along the Governor's Promenade and the reward of a magnificent sight is certain.