Too much distortion when gain staging the acoustic guitar signal level to get sufficient headroom

Have you tried dropping the tracks gain level (knob) ?
You might be feeding a plug in too hot. also verify its not the Audio track itself.
Check the routing to any buses, the master. and also your hardware device/soundcard.

CJaysMusic
The more tracks you have in a song ,the lower all the tracks need to be. Because the sum of 5 tracks is less than the sum of 10 tracks. so check your signal chain and gain stages to make sure your levels are appropriate to your track counts and appropriate to the sound you want for your song. 
 
FYI: You worry about loudness at the mastering stage. in 24bit recording. you just need to be anywhere between -24db and -12dB PEAK and you'll be golden.
In mixing, your peak level should never hit 0dB and your RMS level should be  under -24dB to -18dB
 
CJ

Solid advise CJ.

Yep, until we know exactly what you're dealing with, it's anyone's guess.
 
The solution could range anywhere from no action required to re-record

Simply taking an audio track with no plugins and increasing the volume, even normalising it to 0dB, won't make anything distort until you overload the interface convertor on the way out to the monitors. When you'll get digital clipping which is impossible to miss.

Digital audio isn't like analogue circuits. Analogue gear usually gradually moves into increasing amounts of distortion and compression as levels increase, which becomes more and more noticable as the signal level into the circuit increases. When you increase the volume on a track in Sonar, all Sonar does is add numbers to the 1s and 0s, so there's no distortion created at all.

Obviously, plugins on a track can change this, especially if they are modelling analogue circuits so are meant to distort to a greater or lesser extent. The same applies to non-analogue modelling ones if you overload their inputs.

It has to be said that many, perhaps most, maybe even all, people prefer a sound that has at least a hint of harmonic distortion going on, which analogue mixers, preamps and eqs, tape etc. always create. It's the particular character of the way they very subtly distort the sound by adding or changing harmonics and dynamics that's led to SSL, Neve, Harrison and some other consoles getting the very high reputation they have.