Some days after they venture into Mordor, Frodo and Sam run into an Orc army and are forced into its ranks. They manage to get away quite near to the Isenmouthe, and then the head east beside the road that leads from the Isenmouthe to Barad-dûr. After a while, though, they tire:
The hobbits had gone a few weary miles when they halted. Frodo seemed nearly spent. Sam saw that he could not go much further in this fashion, crawling, stooping, now picking a doubtful way very slowly, now hurrying at a stumbling run.
"I'm going back on to the road while the light lasts, Mr. Frodo," he said. "Trust to luck again! It nearly failed us last time, but it didn't quite. A steady pace for a few more miles, and then a rest."
He was taking a far greater risk than he knew; but Frodo was too much occupied with his burden and with the struggle in his mind to debate, and almost too hopeless to care.
The Lord of the Rings Book 6 Chapter 3: "Mount Doom"
I've emphasized the part I'm confused by. I imagine the risk that Sam "knew" was that they could run into another one of the armies of Sauron, or that they could be spotted and attacked by one of the Nazgûl, so the "far greater risk" must be something even worse. However, on the road they ironically aren't met with any danger at all so there's no indication, to my memory, of what this particularly great risk was.
Why was Sam "taking a far greater risk than he knew" in choosing to walk the road out of the Isenmouthe?