Almost all my tours have ended in a city where I have no friends. Here's what you do:
1. Pick a date that you assume you'll be in San Diego, and add a day or two to it just in case something holds you up.
2. Pick a flight time in the early afternoon, not first thing in the morning.
3. Then book your flight out from San Diego on that date at that time.
4. Look up the hostels and hotels in San Diego ... find one in your price range, and preferably that is close to the airport, and book it for the night before your flight. It's nice to splurge on something a step up from camping for the last night. It gives you a chance to sort out your stuff and pack it, etc.
5. On the day of your flight, get to the airport as early as possible (approx. 4-5 hours before your flight) ... you can cycle over to the airport. As soon as you get there, politely inquire of the airline security where you might be able to purchase a bicycle box. Get the bicycle box, pack your bicycle, and go through the usual airport routine.
Note: you might want to check with your airline in advance if they have bicycle boxes. Not all do. If they don't you'll have to cycle over to a bicycle shop (which you can look up online and contact, now, before your tour), and either get them to help you box your bicycle, and then take a taxi to the airport; or if you don't want to use a taxi, fold the box up well, and cycle over to the airport with the box attached to the top of your panniers. If you do this, you'll have to start early in the morning, which is why I suggest getting an afternoon flight.
Note: You can also sleep in the airport. If you know you can get a bicycle box at the airport, you can just cycle right to the airport the night you arrive in San Diego, get the box, box your bicycle, and then sleep in the airport overnight and catch the flight the next day. You might want to do this if you can only get a morning flight.