Webb could do almost all of it. Meltzer could have, if his bike hadn't nearly broken into pieces halfway through the ride. For me it was too much. I'm a little too old and carrying about 40 pounds too many to enjoy a trail like that in 93-degree heat. After a long climb up off the river, we more or less got close to the top, ran out of water, and out of an abundance of caution against heat exhaustion, got off the trail and took a road roughly two miles back to the parking lot.
Parking is free.
There's a water fountain inside the visitors' center. You get a map for your three bucks, and that's worth the price alone. The trails are patrolled intermittently. Helmets are required. They have a web site, too.