Assuming you run barefoot on the sand your gait and foot strike will be different. This is how you're supposed to run. Replicate the style when you run in shoes on a hard surface. The aim is to make shorter, lighter strides and to land mid foot, so that your heel just kisses the ground, rather than taking the impact of the landing.
This style is harder on the calves though so maybe your technique is okay and it's just the surface being less forgiving. It takes a while to build up the strength in the calves but the key is you can and you'll reduce the chance of shin splints, knee injuries etc. Just make sure you warm up and stretch afterwards. No matter how good your technique, overuse will get you in some way. The strength in your legs to go certain distances will take longer to develop than the cardio fitness required.
Love the concept. Been using it for 6 months. You run much faster also. Got myself a pair of Newton Trainers!