And with this particular race you have to carry your own water bottle so I don't even have to worry about that.
However the down sides of signing up on the day are that it is not a fixed goal to work towards (so I have not been training and also getting on with my life generally which involved an 18km (1km vertical) hike this morning where I got my feet and running shoes wet) and also that there are last minute surprises, if I had signed up in advance I would have known that it was a 12.75 km and not a 10 km (on the plus side at least I won't ruin my 10 km time average...and it is guaranteed to be the fastest 12.75 km race I have ever done!)!