100 km (60 miles aprox ) could be pushing it, I can do 70 miles with nothing, providing I have fed well the night before and a good breakfast, but I still hit a spot about 30 to 36 miles where I "die", after a few more miles I'm right as rain again. Too many rides in this manner though and I do not come back to life again after that 30 to 36 mile dead spot and the ride becomes a struggle.
To sort any problems out if I plan on just water for a ride, I carry a bottle or two of energy drink powder stored in a small breakfast type yoghurt bottle with a screw cap, the name on the bottle looks like "nom", oddly enough one bottle will hold exactly 64 grms of powder which is the maximum i use in a 750 ml bottle. If I find I'm flagging and it looks bad, I just empty the contents into my second bottle, which is still usually full at 30 miles.
Knowing what I know now, I usually plan on one bottle water for the first 20 to 30 and the second bottle energy drink which I start on at about 25 to 30 miles along with the remnants of the first.
