Exercise can help boost metabolism by burning calories. It gets the heart rate up, increases oxygen in the blood and releases endorphins, which all help increase energy.
Exercise also boosts the production of serotonin, known as ‘the happy hormone’.
However, make sure you don’t overdo it as you could reduce your immune system training too hard. Extend your runs or introduce pace progression but keep it reasonable.
And remember, recovery and eating enough are just as important.
If you are sitting down all day then going outside for a walk can give you a vital dose of Vitamin D, from any sunshine/daylight, which will help your mood and boost your immunity.