I am routinely on LI for business, and this set of trails is tops. My only regret is that I didn't find this place sooner. That being said, I was there on a Thursday morning when it was nice and quiet, but my guess would be that weekends are very busy and that may not leave room for runners.
As far as the difficulty, I would rate it moderate vs hard. I am not a mtn bike rider but nothing I ran on seemed over aggressive/difficult. Doesn't really matter...
One suggestion, that someone else mentioned, is to bring some type of map and/or GPS. There are twists, turns, bends, and crossing trails that ALL look the same, which leads me to believe it would be very easy to get lost.
In closing, this is a great place and I will be back.
Trails are marked but if you don't know the route you have to stop and look at map. Many loops come back to other loops and it is not obvious which one you take the first time through.
1. 83North and South are the names of trails, not an exit to the main road.
2. When you hit something like, "Gotta do the ridges" pay attention because when you finish that diamond you will go by the entry point again but the second time you keep going on the blue.
3. When hitting the fork for "no trespassing" and "Iam outa here" you take no trespassing the first time and outa here the second time.
3. Another fork is revenge of the shorty and something else, take revenge of the shorty.
3. When hitting the fork for "hunters path" and dark side of the moon, take dark side first time and hunters path the second time.
4. While doing dark side there is another fork for the "eclipse" and continuing on. Take eclipse the first time. On eclipse you make an unmarked right onto rollers, the second time at that point you go straight. This will bring you back to the fork of eclipse or continuing on. This time you don't take eclipse but rather keep going on DOTM.