That looks awesome!!!! You'll be fine, just get some training, remember to do stretches before & after so you don't end up with any strains, and don't push yourself too hard, too fast. Makes sure you have suitable footwear too. Even on relatively flat shingle easy to sprain an ankle if you don't have suitable shoes.
Do you do a bit of MTB?
"The event consists of a 5.5km trail run, followed by a 15km MTB incorporating the new Altura bike trails, finishing with a final 5.5km trail run".