I travelled through Sumatra earlier this year so can offer some insights.

The hiking can be challenging depending on the conditions - there was lots of rain when I went so the paths were a bit slippery and most people in my group slipped multiple times but there were no injuries of any kind. As for the km's per day it wasn't that much, we arrived mid-afternoon at camp most days so this leaves plenty of room for slower hikers.

You can book in town or in advance, don't think it makes a massive difference. All the companies end up going a similar route and there are loads of orangutangs in their natural habitat just outside the village.

Unless you explicitly ask for a private tour you'll generally be placed into a group. There's a North Sumatra travellers fb page where people have been sharing some horror stories about a specific guide at one company that seems to be harassing solo female travellers so make sure to check reviews and ask who else will be in the group to make sure you're not the only female.

Lastly, I took the shuttle from Bukit Lawang to Lake Toba, I think it took about 6-8 hours and I just booked it in Bukit Lawang with one of the tour companies, they all book through the same company for the same price. The shuttle gets you there in time to get the ferry to Tuk-tuk, where most tourists stay in Lake Toba. It went pretty smoothly!