An analysis of 10 studies involving 1252 people suggests that short periods of "green exercise" (in the presence of nature) has a positive affect on self esteem and mood.
Just 5 minutes of getting in touch with nature, even if in an urban park setting, had the highest payback in terms of time invested and the degree of benefit. More benefit resulted from more green exposure, but the return diminished over time. This doesn't mean that longer exercise periods in the bushes have no benefit; it means that in terms of eliciting an immediate positive response in self esteem and mood, 5 minutes will provide a great benefit.
For fitness and other health benefits, exercise lasting much longer than 5 minutes is needed.
If you're feeling a little pressure at work or wherever, you might choose to take a walk in a local park at lunch to reduce the stress. Other research has shown that moderate to intensive exercise reduces stress (regardless of the environment), so if there is no green space park close by getting out for exercise on your lunch break will still have a very positive impact on your mental, and physical health.
What is the Best Dose of Nature and Green Exercise for Improving Mental Health? A Multi-Study Analysis - Environmental Science & Technology (ACS Publications)