I always carry a spare tire (in case of side wall blow outs) and 2 inner tubes (Continental). I also carry a Topeak Power 16DX tool.

Ortlieb makes a nice waterproof 2.7 litre saddle bag which has a roll closure. Also tape a couple of spare spokes to the underside of the left hand rear wheel stay. Are you using a road or mountain bike? If you are using a mountain bike mostly on roads, get rid of the mountain tires and go for 26 inch road versions instead. If you can fit pannier racks (Ortlieb panniers would be my first choice) this would be far more comfortable than carrying any backpack, especially in the desert heat. Water bottles on the bike would be preferable unless riding in heavy traffic. Good cycle sunglasses such as Bolle are essential. Clipless pedals and shoes are also essential for 20 mile round trips also.