I can typically walk/jog 6 miles. Where is a good starting point to be able to go through the tunnels?

Kendall is 3 miles from a tunnel, so right around 6 round trip

To see the second tunnel start in Wilton, there is an old caboose and parking lot right by the trail

There is a camp ground near the Sparta tunnel.

Kendall or wilton

Tunnel Trail Campground in Wilton, Wi is right on the trail. You can go either direction on the trail and hit tunnels :)