Annie - 7.8kg is a good weight.
My pack weighed just under 5kg when I left home but it fluctuated on the trail, depending on whether I was wearing the raincoat, or longs and a fleece. Some days I wore my Off-Road Crocs on long straight roads as a change from boots, which I then had to carry so it made the pack heavier.
Some people say that weight isn't important if you have the correct backpack but there are other factors to consider. How tall you are, your body weight, your frame size, age, back strength etc.
I am 62 years old, am under 160cm tall (short!), weigh around 54kg and have a small frame. I don't have a strong back or particularly strong ankles and I've got osteoporosis - so I never carry a too heavy load. Between 10 - 15% of bodyweight is about right for me.