The prevailing doctrine states, that you can either win through morale hit by taking the US capital or by victory points, if you manage to keep enough vp sites long enough.
All other approaches seem to fail sooner or later, like an offensive approach in the Western theatres.
The former is normally achieved by an all East (or most East) approach and an attack as early as possible while you still have the generals advantage. The gap between Harpers Ferry and Washington is used most of the time to get behind the Northern lines. Note that this will jeopardize the defenses of your own capital.
The latter has several routes. Most important is a plan to destroy as much lines of supply of your opponent as possible. Defend key positions of your choice or by opportunity and keep the rest of your army moving. Try to fight in mountains, swamps or hills. Make use of the frontage primer here viewtopic.php?p=129422#p129422
Forget about defending the entire coast.
On the long run it is fresh recrutes that make your main bottleneck. Chose your battles wisely. Then don't falter.