It is 20 miles. My suggestion is to do an island tour to see more than just one beach - Cosol tours are the most recommended for St. Lucia. The tour fee is $55 per person. Its 5 to 6 hours. You get to visit the waterfall, volcano, botanical garden, Marigot bay, fishing villages, banana plantation, shopping, beach and snorkling at the Hillton Jalousie hotel which is between the Pitons and many more things on the way. They pay all the entrance fees. Lots to sample of their local taste on the way, and they have a cooler with drinks like water, rum punch, local beers and soda. They leave around 8:30am
Http://www.worldatlas.com/webimage/countrys/namerica/caribb/lgcolor/lccolor.htm Doesn't look very far away on the map, a bit to the south of Castries.
And have you tried looking at tripadvisor site - very helpful.
Hi thanks for the help I went on tripadvisor site and it was really informative.