Honestly I would be very scared, I would try ad survive. I suppose try to buck him off, if it got to bad I would roll and give him my back in hopes of him rolling with me and ending up under me trying to choke me. (its very hard to choke me, I defend very well)
From there I would try and reverse on top of him or w.e to be honest if this wasnt a street fight I would likely have just lost the fight.
Oh man, giving your back is a bad idea in any case. If they dont choke you you dont have much of a defense for anything. You can cover up your temples, but only as long as your kidneys dont start bleeding and your ribs dont break from the punches :/ And even then, if they do get the choke and youg et your chin down you still have to concentrate on all the other blows while kepping your chin down. Very hard position to get out of (probably worse than mounted!)
When you roll, make sure you have ahold of them instead of just rolliong and hoping they roll. Dont let them posture up, grab their neck or shoulder blades and hold on for dear life. If you buck, grab a leg and roll with it or at least try to roll with it.