Start slow and be flexible.
For example, when Frodo decides to take the Ring to Elrond, he takes a shortcut thru the Old Forest. There he has many adventures that don't involve the Ring. It's only when they get back to the Road, when Frodo reminds them, "But perhaps the delay will prove useful -- it may have put them off our trail," does the story turn back to the main plot.
Not ever problem and adventure your PCs have will have to be about foiling the main villain.
For example, when Frodo decides to take the Ring to Elrond, he takes a shortcut thru the Old Forest. There he has many adventures that don't involve the Ring. It's only when they get back to the Road, when Frodo reminds them, "But perhaps the delay will prove useful -- it may have put them off our trail," does the story turn back to the main plot.
Not ever problem and adventure your PCs have will have to be about foiling the main villain.