Originally Posted by nycphotography

Well his heel is dropped in the photo... saddle may be ok height if he puts more toe into it and gets the heels up a little. Dropping the saddle may take away the ability to adjust from calves/toes to hamstring/butt/heels effort when riding.

As to the back & neck, it's probably just core strength / fitness related. I always get neck and back grumbles early reason on my race fit... in a few weeks I'm over it and back to pissing about my legs lol.

OP: Ride more. Do core exercises (situps, crunches, planks, side planks). You aren't particularly aggresive in your position. Get more experience at these new distances and see how things go.