Biking is what you make it.
It can be nice and leisurely or it can be one heck of a workout.
Personally, I'm a big fan of cycling class. In there, you're constantly doing something different, which is awesome for ADD, trust me! I also love the class environment and am motivated by those around me.
I also enjoy biking on a local bike trail which used to be a railway, so it's flat. I'll bike 26 miles on that trail, but again...no hills. It takes 2 hours and I feel great when I'm done. It's definitely a workout.
Biking on hilly terrain is also a great workout and can give you the same benefits in a shorter amount of time.