First of all, great book choice!
- The word hard in the context above means difficult, as in "it is a difficult road to travel on", meaning that it takes a lot of effort to travel on this road.
- Strike the causeway means intersect with the causeway, or meet with the causeway (a causeway is a road elevated about a body of water on an embankment, according to wikipedia). So you can re write the sentence as: 'It goes round the north end of the Marish so as to meet causeway from the Bridge above Stock.'