The local ultramarathoners train here all the time - the longest nonoverlapping route they've come up with on blazed trails is 5 miles.
Since the Albany Running Exchange hasn't had permission to do group runs here for a few years (on trails), orienteering is definitely out...
I was at the pine bush (six mile waterworks parking area) one day that happened to coincide with an ARE Thursday evening event. It was a zoo, the parking area is not sufficient to handle the hundreds they draw. I would guess that's why they can't go there.
I think you were off Pine Bush land and on land which might be owned by Guilderland Elementary School (accessed from Rt. 20). I've found some nice trails in there which likely have been made by mountain bikers. Check my log entry from 9/12/10 for GPS tracks. There are also some trails on non-Pine Bush land between the power line and New Karner Road (accessible from the Red trail I think - you have to cross a small stream on a "blocked" side trail). See 6/10/12 track in my log.
Glen, upon comparing your tracks to mine it looks like you took the right/wrong trail junctions to find the typical mountain biker loops. The official Pine Bush trail map you can find online is very detailed about which parcels are pine bush land and what aren't. It looks like my track was almost all on pine bush land but the mountain biker loops you found are (I suppose) private land.