Hi Alex
Maybe I'm late already with my answer (I'm sorry) - but it might be interesting for other people, too.
It's quite difficult to say - people are different - and so is their "idea" of "moderate" or "hard", too.
Some facts about us:
We're 28 and 32, Swiss, used to hiking in difficult areas. This was not our first trip to tropical rainforests (we did p.e. Borneo as well). We knew more or less what to expect regarding rain, temperatures, leeches (we brought our own leechsocks which were really useful) and so on.
The daily walking distance was less than we expected. We usually started at 9 or 10 am and arrived at the next stop for the night at 3 or 4 pm. Still, the longer trek in Leuser NP is not to be underestimated:
First of all, there is no paths - which means that one of de assistant guides will go ahaed with his knife an clear the way.
Second, the area is not flat. It is very steep sometimes and very very slippery. You absolutely need good walking shoes (no, the usual sneakers won't do it). You will be walking through mud, 1m of greenery, rivers, you will often use your hands and sometimes you will climb, too. Going up can be quite exhausting due to the climate and humidity.
Third, you need to carry your stuff by yourself: This means: 12kg per person - at least. Don't forget to bring enough clothes to have something dry to wear in the evening (you will get wet to your underwear every day) and pack them waterproof. So you need to carry every day: Water (a lot), your sleeping mattress and sleeping bag (you'll need a proper one, a silk or cotton liner is not enough), your personal things. Food will be provided by the staff.
Fourth, there are no "proper" tents. You will be sleeping on a plastic on the floor in a half-open wood and plastic construction. Yes, insects will come inside. So - if you feel panic rise seeing a harmless leech on your sleeping bag - this might not be the right thing for you (though the guides will appreciate your screaming :-))
On the whole I would rate our trip moderate to hard.
I don't want to put anybody off from doing the trek. We loved it and it was by far the most beautiful trek we've done. But I think one will need to to jump over his/her own civilized shadow to really enjoy. I will never forget the evenings we spent in the rainforest, surrounded by fireflies...beautiful!
Just one remark concerning the food: The food was fresh an wonderful. Our guides picked fresh jungle vegetable every day and we ate freshly caught fish. This is what I call truly organic food! We had fern shoots and hearts of palm, we had sticky rice cooked in bamboo over the fire...the best food we had on our whole trip on Sumatra :-)
Enjoy!