Boundaries of large areas are usually mapped as multipolygon relations.
For the national park, you would:
split the area into shorter ways
create a relation
add at least the 3 following tags: type=multipolygon ; boundary=national_park ; name=Alpine National Park
add the outline ways as members with the role “outer” to your new relation
remove the name and boundary tags from the outer ways
It seems that MikeN was faster to reply, but our postings mostly agree anyway. There are only a few small things I’d do differently. The advice to check whether the ways have more nodes than necessary is useful, but don’t blindly use the Simplify Way tool - it’s often overzealous and deletes relevant information. As for leisure=park, that tag is normally used on smaller parks in cities rather than national parks, a distinction that is explicitly mentioned on Tag:boundary=national_park. Also, using wikipedia=**en:**Alpine National Park as the Wikipedia link would avoid ambiguities by explicitly linking to the English edition of Wikipedia.
Any polygon can be expressed as multipolygon according to OGC Simple Features specification (which OSM does not totally follow). Polygon is limited to have one outer ring and zero or more inner rings while real multipolygons have several outer rings. Most OSM multipolygons have only one outer ring and some holes so they could be called just polygons but at the same time they are also simple multipolygons. Wiki page http://wiki.openstreetmap.org/wiki/Relation:multipolygon does not have an example of a multipolygon relation with only one outer ring and no holes but I think such a thing would suit your national park well.