Baytril is a good antibiotic drug to start on. There are other antibiotics than can also be used. You should be injecting it. Make sure you follow the instructions for injection. Baytril is not nephrotoxic and you can inject pretty much anywhere.
Rear legs are fine.
Switch legs for every injection. Be careful not to poke the bone. For respiratory infections, Baytril can be given for several weeks. Minimum, as for all antibiotics, is 7-10 days. 2 weeks is a good time frame for a first time.