You can take the buggy over to the first continent. Drive it into Costa del Sol, and you will actually enter the town. Return to Junon via the Cargo Ship. When you leave Junon you will have the buggy with you. Since it can cross rivers at shallows, you will be able to reach the Old Man's cave. Once you have obtained the Mythril, you can drive the buggy into Junon and return to Costa del Sol.