This is an easy hike whose distance can be tailored for a number of objectives or difficulties. To get to the trailhead, take US 93 toward Hoover Dam and turn left into Lake Mead National Recreation Area where the Alan Bible Visitor Center is. (The VC is currently closed for renovations). Parking for the trail is on the right side of Lakeshore Road just past the Visitor Center entrance. The trailhead is before any fee station if you come this way.
The tunnels were built in 1931 to carry all manner of things needed in the construction of Hoover Dam. The hike is generally level for the tunnels, but if you go on to Hoover Dam, there are some ups and downs to do.
When you come out of the fifth tunnel, you have three options. Option 1 is to turn around and hike back to your car. That will make a total distance of about 4.5 miles.
Option 2 is to do a little exploring in this area. The fifth tunnel is under the scenic overlook parking near the security checkpoint. You could climb up the bank and enjoy the view. There are no benches or tables, but you can sit on the rock wall.
Option 3 is to continue on the trail which goes right to the parking garage at Hoover Dam. Along the way, you can take an Option 3a. That would be to cross the road going to the dam and enter the parking area of the O'Callaghan-Tillman Memorial Bridge. From here, you can walk up the stairs to the bridge, walk the sidewalk across the bridge and back, and then hike back to the trailhead. I have that trail with GPS coordinates on my EveryTrail page at http://www.everytrail.com/view_trip.php?trip_id=873657. This would make a round trip distance of 8 miles.
Continuing to the dam and back without going to the new bridge would give you a total distance of about 7.5 miles.
The nice thing about this trail is you can also ride it with your bike. I've done the trail about 5 times hiking and 1 time with my bike and it's a great bike ride.
Check out the website in the listing to read some interesting history of the tunnels and their construction.
Happy hiking!