Depending on your riding style and fitness, I would doubt that most e-bike batteries would last the distance of a 100k. Jan and I have found that in Essex, on our tandem, the battery is good for up to 40 miles or so, more like 30 in Pembrokeshire. Most of the assistance is on the hills. This can be eked out with judicious use of the on-off switch, but we haven't really experimented with that a lot.
OK, you may well be fitter than we are (most people are!) so the battery might not cut in quite so often (they stop helping at about 24kph) giving you a greater range, but the less electricity you use, the less reason there is for you to use an e-bike!
Our first group ride on our Helios tandem was the Andy Allsopp Birthday Bash, which was very hilly in places. It was around Bromley and bits of North Downs Kent, and our overall average was about 10 mph, a good 2 to 3 mph more than we would have expected from our Thorn tandem.