There are a ways to go about this but in this case I've solved it by creating a new gravel patch that contains a footstep imprint. This patch can then be use inside another Forest object and each step placed manually using tree edit mode
In this way we can use small individual gravel objects to get the required definition within the patch, but then place patches to avoid having to individually scatter millions and millions of pebbles to fill a large area.
Then, to fill in the remainder of the area, add a new forest object and pick a corresponding gravel preset designed to fill a large area. Open the Areas rollout and add the Footsteps Forest object shown in the screen grab above as an exclude area. Adjust the scale value to slightly overlap the two forest objects.
In the screenshot above the footsteps are shown in red/purple and the main gravel area is in green. Note how the two slightly overlap. The final result looks like this. The footstep impression is slightly exaggerated for clarity.
Because the two forests are referencing one another, the nice thing about this technique is that you can easily update the position of the footsteps and the large area of gravel will update automatically.
I hope that helps, please take a look at the attached file and let me know if you have any further questions.