1. They can outrun the black riders.
2. The black riders alert the witch-king.
3. They can’t outrun the fellbeast.

So yeah, pretty short. But if they are able to maintain stealth, then:

1. They get stopped at Caradhras by bad weather anyways.
2. They go through Rohan instead of Moria (otherwise they’d have to abandon the SUV or be very dangerous.)
3. They outrun any of the cavalry that might try to stop them.
4. They get through Minas Tirith months before the battle, no biggie.
5. They get stumped at the Black Gate.