Q: How important are recovery days for recreational athletes? Is there a difference in recovery for someone training for her first 5k, and someone training for her first marathon?
A: There are two types of recovery - passive and active. Passive means rest with no physical activity. This is generally best for novices. Taking a day off from exercise will allow the novice's body (and mind) to recover and grow stronger. Working out will almost certainly be too fatiguing for the already tired novice athlete.
Active recovery is often better for the experienced and highly fit athlete. This means doing a short, low-intensity workout. The intensity part is easy. That means zone 1, for example, if using a heart rate monitor or GPS pacing device. A "short" workout may vary greatly between athletes. For someone training 12 hours per week "short" may mean 45 minutes. For another athlete who does about 4 hours weekly "short" is more likely something such as 10 minutes. Experienced athletes also need passive recovery days from time to time, just not as frequently as novices.
If in doubt about what to do - passive or active, 45 minutes or 10 minutes - be conservative. It's always better to err on the side of too little rather than too much when it comes to training.