If you do single track, you won't be minimizing chance of injury.

I would just pedal around and enjoy yourself. Ignore average speed and distance threads here like the plague.

Your body will let you know when it's bored and wants to be pushed a bit harder.

Most here will tell you it take about two years to acclimatize to your bike. I think that sounds about right.

The best training device is a.....camera. If you carry one and stop and take pictures of interesting stuff you see on rides, it will keep your speed Ina safe range and make your posts a LOT more interesting.