Almost all of the approximately 101 miles of "wall" that have been built so far, replaces existing infrastructure.
The lengthy procedure to acquire permission to construct a barrier at these locations has definitionally already occurred, and these locations are more likely to be deserving of a barrier (otherwise, why would a barrier already be there?)
This photograph provides an example of "wall" being constructed to replace existing vehicular border infrastructure:
And this photo shows new "wall" being constructed to replace existing pedestrian border infrastructure:
This site gives detailed geographic information about the construction of the wall.
It claims 101 miles of wall have been constructed, but does not not break-down this figure according to where fencing pre-existed.
It does, however, include a map that shows where contracts have been awarded, whether they replace existing infrastructure, and their completion status.
The majority of the wall appears to be replacement for existing infrastructure:
The website shows that contracts for 292.75 miles of wall have so far been awarded to construction firms.
The total length of the US southern border is 1,954 miles. According to one source, 130 miles of border has topography that makes artificial barriers unnecessary (eg. river crossings), leaving 1,824 miles.
If these figures are correct, 16% of the total border length is currently covered by a "Trump Wall" contract.
That said, it should be borne in mind that ease of crossing will vary considerably along the length of the border - even a partial border wall will likely have an impact.
Presumably the focus for construction has been driven by considerations such as:
- Availability of federal funding
- Prevalence of illegal crossings at each border location
- Legal status of land at each location, and
- Topography, including presence of natural obstacles (rivers, mountains etc)
The following photographs show some of the existing border infrastructure. Pedestrian and vehicular.
Update, Feb 2020: the Pentagon has announced it will divert USD 3.8 billion from previously allocated military funding, for wall construction.