have done a couple tours (1000 or more miles) and from personal experience I can tell you that the road rules are basically as follows
First of all, by highway, I am going to assume that you mean a freeway. There is a difference. Freeways are always at least four lanes and have no cross traffic. That is, roads that cross freeways do so using either under or overpasses so as not to interfere with the traffic on the freeway. Additionally, freeways must have some kind of median separating the lanes carrying traffic in one direction from the lanes with traffic going the opposite direction. On the other hand, highways can be only two lanes, can have cross traffic, and don't have to have opposing traffic separated by anything more than a line painted down the middle of the road. A freeway is a special type of highway, but a highway is not necessarily a freeway. It is usually freeways where there are restrictions as to the type vehicle that can be driven on them.
That clarified, the general rule is that if there are surface streets that the bicyclist can use instead of the freeway to get where he is going, then he may not ride on the freeway. For example, if you are within an urban area, then there are usually surface streets that a bicyclist can take to get to just about any location in that city. Such streets may not be quite as direct or convenient, but they are available. So, in urban areas bicyclist are not allowed on freeways. On the other hand, in between urban areas, there may be not other streets that the bicyclist can use. So, they are permitted to use freeways to get between cities.
In California (where I grew up), in urban areas, the freeway onramps usually have signs specifically stating that bicycles aren't allowed on the freeways. The on ramps at the edge of towns on the out of town bound lanes, however, lack these signs. However, the off ramps for the townward bound lanes in those same places will have signs telling bicyclist that they must exit the freeways at those point.
So, if there is a reasonable alternative to the freeway that the bicyclist can use to get to his destination, then he is not permitted to and should not rid on a freeway. However, if there is no reasonable alternative, then the bicyclist may ride on the freeway.
hope that can be of some help to you. as for packing that drastically varies from person to person. just pack as light as you can and bring only bare essentials, try to find bike shops on youre route to lessen the load of tubes and tires and energry bars and whatnot. any more specific questions feel free to ask!