This is coming from the perspective of a 39 year old who hadn't ridden a bike since his college days, and back then it was always on tarmac. After a Friday evening of getting reacquainted with balance and eventually jumping curbs I decided to give these gateway trails a try. My 12 year old daughter, who also has never ridden off road, joined me.
Hazel Dell Landing is definitely easier than River Road. Entry is South East from the parking circle, very close to the river. Don't get mixed up by the Frisbee golf trails. There is no markers pointing the trail out and if you don't know what to expect because this is your first time it's quite easy to find yourself heading the wrong way. The trail is not a loop but rather a single trail with a turn around loop at the end. This means you can run into people going either direction on it. The trail follows white river and takes you quite a distance from the parking area. There's very little in the way of hills save for a couple significant gullies. There's no riding next to a ledge or anything that might make the new rider nervous. There's 1 tight squeeze between 2 trees that I held my breath going through. Biggest hill is probably 15 to 20 feet and both my daughter and I navigated it easily. Other than a few fallen trees we could have easily ridden the entire trail without dismounting. On 5/4/2014 we encountered no other riders during a 2 to 3 hour visit.
River Road Park was surprisingly harder. From the parking area follow the paved path and take the left at the fork. You'll have a field on your left and you'll see a trail entry sign on your right as you near a park bench. There is quite a few other entry points as well. The trail is a complete circle with an upper level and a lower level. People can ride/walk in either direction. It was pretty evident that counter clockwise was the norm. You are also quite close to the parking lot and bathrooms no matter what part of the trail you find yourself on. On 5/4/2014 we encountered quite a few people using the path for walking or riding, all of which were very nice. This trail has quite a few more elevation changes (albeit pretty small changes) than Hazel Dell. The entire trail is built into a fairly steep hillside and often the trail brings you quite close to the edge. Both my daughter and I found it quite unnerving on our first ride through. On our 3rd go we were pushing a lot harder and no longer found the edge bothersome.
All in all, Hazel Dell was a very enjoyable ride that was fun but far more relaxing. Most of the time I felt like I was just biking on a path and not mountain biking. River Road was never relaxing, it had my adrenaline flowing and kept me challenged even on my 6th go around. I was able to put to use advice I had read on this forum and by the time I left I was a much better and more confident rider.
Hope this helps someone who's unsure where to start.