Will marinating chicken for 18 hours make the meat fall apart? I decided to marinade some chicken for tomorrow night's meal but the only time I have to do it is tonight. Obviously it's going in the fridge for that time period.

I've never marinated meat for that long before and I'm concerned the acids in the orange juice marinate I'm using will break down the meat too much and it'll be all mushy and gross.

Any experience or thoughts on this?
It'll be fine.  I've done this multiple times, even occasionally for more than 1 night and never had a problem, if anything it improves things as the marinade has time to work into the meat.  

If you can, give it a stir a coupe of times (every 6 hours maybe?) to ensure even coating.  

The acids in orange juice and most (?) marinades are generally too weak to break down chicken into component parts.