Bike Path! My Kingdom for a Bike Path!

I don't think this qualifies as long but it is better than nothing...

http://www.chandleraz.gov/Content/Paseo_Trail_Map.pdf

It is easy to park at the tennis courts at Tumbleweek park and then go to the trail.  There are bathrooms there... I ran it last week and was passed by several bikers...

I think Riggs is your best bet.  It's long, hardly any traffic, and a huge shoulder if you need it.

The shoulder is a bit rough, but there's so little traffic that most people just ride on the side of the road.  The loop around South Mountain is pretty good too except for the leg down Baseline.

Just go down Riggs and it turns into "Beltline Rd", which heads North.  Beltline Rd then turns into 51st Ave.  Just stay on 51st Ave all the way to Baseline.

Baseline S_CKS !  Tons of traffic, and the populous there have shown to be very biker unfriendly.   Send "hmjiii" a PM and he could send you a route that takes a bike path that skips most of Baseline.

Another South Mountain option is Silent Sunday:

Silent Sunday will return each month in 2008!! On Silent Sundays, bicyclists, tots in strollers, in-line skaters and wheelchair users get to call the Park's roadways their own. (Closed to cars). Starting at 5 a.m. the park's roadways past the Central Avenue gate* will be reserved for non-motorized use. 2008 Dates: Feb. 24, March 30, April 27, May 25, June 22, July 27, Aug. 24, Sept. 28, Oct. 26, Nov. 23 and Dec. 28

The loop around South mountain is pretty good actually. You head down to Riggs and follow it like Cordova said until it turns into 51st Ave. Then you go up to Dobbins (Not baseline) and take that clear over to 20th St. From there it's tricky. You can go up to the Highland canal trail or up to Baseline or Southern. I recommend the canal followed by Southern, and only Baseline in a dire situation. The canal's a little bumpy, but it's paved all the way to Baseline and 41st St. From there you have no choice but to take a major road like baseline over to 48th St, but it's only for a mile. You can take 48th or Priest back down into Ahwatukee. 

Here's a map: http://www.sportsfun.com/gaba/maps/AroundtheMtn3.doc
It's a little old, but combine it with the directions I gave above, and you ge the idea.

Gaba is a great group and website for these routes: http://www.sportsfun.com/gaba/ridemaps.html
Most of them start from Desert Breeze Park in Chandler.