I am 33... and it usually takes me about 5 miles to get into the zone. Which equals about 20-25 minutes.

Then, if I stop, I have to get warmed up again... even if I have already done 10 or 20 miles.

And, when I have done gym workouts... I tend to really start feeling it about 30-40 minutes into it.

Here's how I see it though... I was built for distance... endurance... and someday that will get me through a 1/2 Ironmand and then possibly and Ironman.