Long lines: whether or not to treat PCB traces as RF lines
When considering traces as transmission lines for digital signals, switching frequency does not matter. 

Forget about your MHz. Or kHz.

Only slew rate matters.

Detailed explanation:

A driver is connected to a transmission line. It switches from 0 to 1 in a time T. The waveform propagates along the transmission line, reflects at both ends, and the original signal combines with its reflections at the receiver.

If the edge transition time is shorter than the time it takes for the reflections to dampen, then the received signal can contain errors, no matter its frequency. Even if it is a 1kHz clock to a shift register, if the edges are doubled, then your shift register will shift two bits instead of one.

This is the first signal integrity aspect. Check your part datasheet for slew rate, or measure it with a scope. It's a quite slow microcontroller, unlikely to be equipped with super-fast IO buffers. Your board is small. You should be absolutely alright IMHO.

The second integrity aspect is ringing, due both to reflections and LC interaction between trace and pin capacitance. Having IO buffers with lower drive strength dampens it. Your uC datasheet mentions something like 10-20mA drive strength, so output resistance is not negligible. It should dampen it.

In other words, you shouldn't worry unless you're driving cables.

PS: Don't forger about EMC and grounding.