I've had TONS of practice against AT-Ats in Shadows of the Empire, and the mechanics are essentially the same. But when you first hop into the speeder, make sure you use the Pile In command to get a gunner. If you don't have a gunner it's hard to fire the tow cables.
I would use the speeder on hoth, take down the AT-AT's, clear out the AT-ST's then provide some more support for your ground troops until more AT-ATs or AT-STs repsawn.
And use the caves inside echo base, there is a base that has a cave which leads to Echo Base, if you lose that base then you essentially lose the war since you lose your Speeders.