**** You don't have to head to the closed off destroyed bridge, follow the detour*** look at the hint perhaps
The walk to the cache is about 1 mile from the main parking lot it is mainly flat
****VERY IMPORTANT PARKING UPDATE, you have to reserve a spot (for fee) or take the shuttle, please check out the link below, if you are more of a hiker you can always hike in from the top for free, from the pantoll area (parking fee) or park along the road going up to pantoll, there are trails going down from there and road side parking,
You can always come down bootjack and other trails for free, fee only charged for coming in the main entrance.
Access to the cache is from about 8am to 5pm - 8 pm depending on the season, check out the link for the muir woods
http://www.nps.gov/muwo/index.htm
Threre are many hiking opttions out of this park (and one of my fav's is going up Bootjack trail and alpine to pantoll, ranger station, then old mine trail to dipsea trail, dispsea trail down to the right, steep ravine trail turn left, a short distance on this trail, half way up the next hill turn left and follow steep ravine down to Hwy 1 cross Hwy 1 carefully and walk down the road to rocky point beach (there is another of my caches, accessible by low tide only) I prefer this rocky beach or crowded stinson beach, retrace your step and go up steep ravine all the way to pantoll (steep ravine is a pretty trail) , then find the way to ben johnson trail and back down to the muir woods , that's around 14 miles and think around 2500-3000 feet elev gain.
Map for Mt Tam, here is the link, (map on page 5)
http://www.parks.ca.gov/pages/471/files/MountTamalpaisSPFinalWebLayout110816.pdf
And here is a link to a great multi cache by Lab e rinth
http://www.geocaching.com/geocache/GC19KQM_opening-the-mountain
it's a 15 mile hike and requires to find a bunch of caches in order to get the final cache