For you next year, given what you've told us, if everyone is expecting you to march at UIL this coming fall....that is flat out crazy and unrealistic to the Nth degree. Having a non-competing marching band doing 10 or fewer sets would be possible with super easy music & drill...although still a big challenge. You know your kids better that anyone else here, so that is up to you.
Since they are your biggest supporter, have a sit-down with your principal....now....don't wait for after TBA to do this. Explain to them exactly what things need to be in place, both on your end and on the school's end, in order for a marching program to be respectable. Also explain that at this juncture you are going to need a minimum of three years, and probably five, before the marching band will be ready to go to UIL...especially the fact that strong concert band performances will bring strong marching performances.
Like I said, have this conversation with your principal right away; Tuesday if at all possible. The sooner you sit down with them, the less likely you will be seen as an excuse-maker, and more like someone who knows what the situation is now, and has a realistic, long term plan to fix the situation (and stress that it is a long-term fix).