Only sport gliders carrying a maximum of 2 people achieve those speeds fuel free. A large, heavy cargo glider carrying a full load is just going to crash without a tow rope, regardless of winds or thermals.
You probably wouldn't go totally fuel free - you'd go mostly fuel free.
And yes, the craft would have to be absolutely huge for a decent payload - and it might make sense to use different construction techniques considering that - for example an inflatable fully pressurised craft.
Could probably add relatively inexpensive drone systems to the gliders with some solar and a battery to enable it to assist in landing and taxiing, as well as dealing with weather and the inevitable crashes.
I am aware that flying is the safest form of mass transportation but crashes do still happen and should be accounted for, after all, and it would be nice if the autonomous glider drone systems had a touch of brains in it to try to not land on a house or into a highway or something.
And yes, the craft would have to be absolutely huge for a decent payload - and it might make sense to use different construction techniques considering that - for example an inflatable fully pressurised craft.