The first thing I did was put the smallest front chainring on my bike that I could. I think its a 12 tooth ring. That is a neccesity for pulling these types of loads. That way I can spin efficiently while going as slow as 4 mph. The fastest I can go on straight aways with a huge load is about 10 mph and on hills it can be as slow as 3 or 4 mph with a monster load.
The largest load I have had on that trailer is about 400 pounds and at that weight you really need to watch the hills. There are some hills that you just cannot turn the crank arms anymore.
I try to avoid hills that are greater than 5% and since I am friends with the local traffic engineer I can find that information fairly easily. I have taken on steeper hills if they are pretty short.
Braking I have not had any problems but I chalk that up to the disc brakes.
The furthest I have hauled a heavy load (300 #'s) was 20 miles round trip with about a 2 hour break in between. When I returned I was very tired and after dropping the trailer off I thought I had bent my frame because the bike felt so weird to me. I think what happened was that I ride with a lean while that trailer tongue is pushing down on my bike and after removing that it felt very strange.
My town is pretty hilly but I think if I lived in a flatter area then I could haul more weight or go a further distance.