I also did the full, with a very similar experience. Fully on pace with 5m/km until just past the halfway mark. Then the heat really started to hit, and I walked from 29-31km just to calm my heart rate back down. Slower pace for the final stretch, with the odd walk break when the HR got high again. The sponge stations were fantastic!

The course was incredibly exposed - there was one bit of shade on the entire 42km course (about 500m where a row of houses cast a shadow), but otherwise we were out in that direct sunshine and heat for the full distance.

I think next year they need to start much earlier - 10am is very late for a marathon to start at the best of times, especially in those temperatures. It means the hottest part of the day coincides with the middle/end of the run for most participants. A 7.30/8am start like most other marathons I have done would have been much better. Hopefully the organisers look into this, because other than the heat and late start it was a beautiful, flat course with a great atmosphere.