John,
There are two ways to look at San Gorgonio...Vivian Creek and South Fork. Vivian as you state is 16 miles RT, +5,960' but South Fork is 22 miles, +4,600'.
I love the Vivian Creek Trail as I love the South Fork Trail doing this both ways within your training regiment. This gets you the gain and distance.
With that said, you have to advance permit, the closed at this time, Vivian Creek Trail. They have not allowed self permitting for this trail...meaning you will getting awfully late start if you walk in at opening and get a permit.
Personally, I like the San Bernardino Peak Trail better...or the Forsee Creek Trail. It's less traveled, prettier and you can go out to San Bernardino East or Anderson and get comparable distance and gain.
