wow, LS...you have one fit Chihuahua!! Its worth noting that not all Chihuahuas can manage that much activity.Adam will happily do about 5 miles a day but more then that and he gets grumpy and too tired.
Mine are 3.5yrs and 2.5yrs. We do anything from a 30 min road walk to a 90 min road/field/park walk in the morning.
Then usually a 30 min road walk in the evening, although if my mind wanders it can be an hour!!LOL
Sometimes we go out for a long walk (so 3-4 hours) but then we cut back the exercise the next day.
It always amazes me that people think Chihuahuas can barely manage to walk to the end of their block without collapsing!