Hi,
Early on we did mostly rail/trails and very low traffic routes. Maybe 15 minutes - 30 minutes around the neighborhood. It also is an excellent stroller, so the kids can get use to it. It does impact you speed but not as much as a single. You can still go plenty fast downhill, but feel the trailer going up. You need just a little extra for rolling hills.
We use the flag and it sticks up about 6' so I think that helps, plue the trailer is yellow, so between the large bike (tandem), flag, and trailer, there is a lot more to see then a single bike. We try as always to stay away from heavy traffic.
We can easily ride 1 - 2 hrs with our son, maybe longer with decent breaks. Our daughter was a different story as she would get bored more quickly. Our son will actually fall asleep in the trailer, so that is nice. Often he is saying his ABC's or singing Bingo to the point of driving you nuts, but often he just eats a snack and plays with his toys.
I would avoid a cheap trailer and get a Burley, there are tons on ebay now. We'll likely sell ours in the Spring, after we are sure he is ready for the tag-a-long, but it's well worn. The Burley trailer folds nice, is light weight, makes great strollers for 2 kids, plus it's very well made.
John
This is my wifes mtb pulling the trailer:
