A couple of people asked me this today.
Spit specific:
When walking from your car to the launch area, carry your board with your left hand, with the fin pointing in the direction of travel. The mast extension is shoved through the right footstrap. Carry your rig with your right hand.
When walking from the launch area to your car, carry your board with your right hand, with the nose pointing in the direction of travel. The mast extension is shoved through the right footstrap (same footstrap as when carrying from your car to the launch area). Carry your rig with your left hand.
It's extremely easy to do. One trip. Secure. You can walk a kilometre like this like it's no big deal. Seriously. Unless you try it, you'll never know how easy it is to do.
I actually use my outhaul line to lash everything to the boom, and wear my harness. I'm sure you can figure out whatever tweak suits your situation.