The D&R Canal Towpath is quite flat, which is not surprising when you consider that it runs alongside rivers.
As you can see in the elevation chart below, the elevation at the western end near Milford is around 125 feet,
and it drops to around 15 feet at the eastern end in New Brunswick.
Remember, this elevation drop (or climb if you're heading westward) is spread over 70 miles,
so it averages out to about a foot and a half of elevation change per mile.
That's so gradual that you should not expect to notice it in either direction.
In the chart below, the paved sections (mainly around Trenton) appear as solid, while the unpaved majority of the trail is striped.