Maybe there is another very important point to be made at this juncture.
The prime responsibility of any rider when setting out on a trail ride or hack is to get him or herself AND the horse back to the stable yard safely.
When one takes up the reins, one assumes the overall responsibility for safety.
It is fun to test one's riding ability, but not at the expense of risk to the horse.
The human makes his/her own choice, the horse invariably has to do what it is told.
If the rider puts the horse in a situation which is beyond the capability of the animal either for lack of fitness, lack of schooling, temperament or capability then any injury to that horse would be due to the rider's incompetence.
Down hill riding is one quite dangerous pursuit - if the slope looks too steep and if it cannot be negotiated in zig zag fashion, then that cannot be the place to descend. It is so easy for a horse to slip, thereby break its leg and fall on the rider - serious injury even death could result.
By all means start with short , gentle slopes, but don't ever assume that your horse is infallible. Remember the rider is the guide of the horse - the load falls upon the horse to carry out the manouevre safely for both equine and human.
This responsibility issue is often forgotten in competitive scenarious - never overface your horse.