we have just gone back to being able to go for a bit of a walk outside with Noah in the stroller. at the age of 2. I decided that I wanted to get some exercise, so now we walk to fun places like the "diggers", the dogs or the park etc. but I try to walk for 20min or so before letting him out for a bit. all in all at least I get a good 45mins of walking in or so.
I have a chariot stroller---and they really are awesome, we will upgrade to the double when the new babe comes.
I figure that as he gets older, he will be more inclined to sit for longer.