Giligan was about 240 meters long

Holy shit that is a giant dolphin. ;)

Regarding fungi being used to fix infrastructure, it's not intuitively beyond the pale, although in practice it seems very difficult. The final mycelial structures of some fungi can definitely be rigorous. The company ecovative is already making a variety of mycelium based products, including biodegreable storage and packaging materials. Plus healthy mycelium tends to be pretty damned comprehensive in how it grows, so filling up small gaps would be right up it's alley.

But even if there's a mycelium species that is strong enough for this application, it's not clear to me how they would foment its growth? Fill the gaps with some kind of nutritional medium? Plus the mycelium and fruiting bodies of some species can actually destroy certain manmade materials. There are photos online of Shaggy Mane mushrooms, C.comatus, breaking out from underneath solid ashphalt.

So, definitely seems to be in the realm of pure brainstorming at this point, but I will add it to the list of fungal projects folks around the world are considering.