I'm not sure how many miles there are, but you can follow nice crushed granite paths from Sewell Park all the way to the San Marcos River Pub.
Shully Canyon ( I hear ) has some really nice bike trails...I've never been there, and not sure exactly where it is, but I'm guessing at the end of Shully road up behind campus.
There's a new park off of Prospect street, which is off of Bishop just after you turn from RR 12/Hopkins.
I take the dogs out there alot. No real trails set up yet, more like foot paths for now, but def. ridable with a decent bike. Cool thing is....there's never anyone out there and the park is freakin' huge, with no development YET...
Playscape Trails....there is a large childrens playscape, right on the river, infact the trails I mentioned earlier go right past it. Anyways...if your driving and you turn into the lil road that goes back to the playscape from CM Allen. After you cross Purgatory creek, and before the playscape...the woods on your left have a bunch of homegrown trails, jumps, and whoop di dos. Built and used by BMXers but it's fun to tool around on,,,for me, with my old ladies walmart special mountain bike..
Screw Austin...There's plenty of places to ride here in San Marcos.
Off POst road, going out Lime Kiln, behind the elementary school, there is a large park out there with some cool trails.
And if you really want to get off road, and in nature, my buddy leases a 400 acreas ranch off Hillard. Throw him a donation, and I'm sure he'll let you ride out there.
And probably the best spot..but I don't know cause i've never been there is the Ranch that the University runs..can't think of the name right now...but Ive heard that there's alot of trails out there too and you can ride along the Blanco.