Well, 12 hours marching time at 3.5 KPH divided by 1.6 puts them about 26.5 miles from town. How many more miles is it from the river to the town?
Looks to me like you could destroy the bridge, move all the boats to your side of the river and wait for the Army. If the river is too wide and fast to ford without the bridge, they'd be hard pressed to get a credible force to the other side before the professionals arrived.
Your pilots could keep an eye on them without getting in small arms or RPG range. If they don't have any logistics with them, they'll have to stop for water and rest at the river anyway. They'll also be kind of hungry at the end of that 12 hour forced march.
Some security forces between the river and the town in case they do manage to cross the river will force them to deploy into a tactical formation and this will slow them down considerably.
Even if you just dropped one span of the bridge, they'd be hours rigging a crossing, especially if you covered it with sniper fire.
That's my plan, drop one span of the bridge, cover the obstacle I've just created with fire and let them stack up into an easy ICM or MK 82 Rockeye target on the far side of the river. It'd be a turkey shoot once the Army arrived.