Fear made him keep going, but even fear couldn't keep him running forever.

Clifton's sense of humor is that he will keep a joke running forever.

It was clear I couldn't keep running forever.

Am I going to keep running forever?

He had been running forever.

The Halting Problem asks whether a computer program will stop after some time, or whether it will keep running forever.

You have to keep the greenhouse up and running forever or the flowers will go extinct...but you never...

He seemed to be running forever through some horrible dream, trying to catch up with something ahead or eternally fleeing from some disaster behind.