Day 2: So as with most plans, it didn't survive contact with the enemy. The enemy in this case being a power outage that lost me a few hours of progress. Wasn't as big a deal as I had initially feared however, as I had only really been grey boxing out my lower levels and after taking a break I was able to get back to it.
I've only gone with two floors, rather than the three I had planned as I just kind of ran out of ideas of what to put into all that space. I have a good idea of what I want the flow of the level I've got to be, and I can always come back and change it later.
Plan for Day 3, 4, 5 and 6: Well unfortunately I'm back to working twelve hour shifts over the next four days so will get very little time for deving. Hoping I can at least search around for some textures I can use for the walls and floors. On Day 7 When I'm back, the plan is for me to add those textures to the level. Get rid of the faces I dont need so the tunnels actually connect, and add some lighting to my level.