At a total distance of ~67 miles this is the furthest trip I've taken. I took along a single large 48V 20AH Lipo pack (around 900 watt hours). I pedaled 95% of the way down, and pedaled/ebiked back, and I still had around 200 watt hours left. It took around 3.5 hours to get there, and 3 hours to get back. I limited the max power (using the Cycle Analyst 3 controller) to 450 watts to conserve energy. 450 watts makes for a very gentle ride.
Here's the route I took on mapmyride.com. Note on the way down I took a slightly different route to see the Cottonwood Creek Trail in north Dallas. This route is almost entirely on trails. I only had to spend a few minutes on low traffic roads to "bridge" between the trails.