Take a city bus to the airport (or the metro when it will be completed). Walk back until the gateway of the airport (a few hundred meters before the international terminal), and take the parallel road. Walk past the viaduct over a crossroad, keep heading South. After 20 minutes, you'll see the pay-toll.
Note : right after every pay-toll, there is a large bay where drivers going a long distance often stop to put money in boxes to bless their journey. Therefore it is very easy to catch a long-distance ride to virtually anywhere in Southeast Iran and Tehran (ex by Olivier : direct ride to Tehran (900km) in less than 10 minutes !)