Sorry to hear about the slide, but glad you and the bike are okay!
Regarding the patrol cars showing up...(expletive deleted) nanny state!
Are there any "no trespassing" signs posted on the airfield? If not, I'd have had half a mind (there's part of the problem right there, lol!) to tell the cops that you are essentially doing nothing more than refining the skills -- off of the public roads, mind you -- that are on the DMV test to earn the motorcycle endorsement
so that you can maintain proficiency on your bike, especially since the DRZ you are riding is a new bike to you!
But then again, I don't take to authority very well, and I seem to sometimes find myself on the wrong side of the valor vs. discretion curve, so I'm probably not the best guy to listen to