It you REALLY want a long distance flying glider, then look at electric.

Learn to RE-wire a standard 3 pole motor to get more torque. Spin it slow. This will require higher voltage (11.1V li-po). A 180 size electric motor or even a 280 size will turn MORE than enough prop to keep it afloat. You don't need screaming RPMs like a glow engine. You just need enough SLOW thrust to move it fast enough to make it fly.

Rewind a small electric motor for torque and slow speed. Use an 11.1V li-po about 1500mah.

It'll fly 3hrs or more. Guarantee it.

It'll be lighter than even the smallest glow setup. You won't have to worry about fuel delivery. You won't have the hassle of assuring the engine runs for the entire tank of fuel. Electric is SIMPLE and RELIABLE. Why do you think any idiot can buy a park flyer and go flying? Because they are electric and there is no brain power involved in making them work. Plug it in and fly it.

Hint:
For tips on rewinding electric motors, see rccrawler dot com. Do your homework. It can be done.