Running

Start slower than you think you need to - not just in terms of pace but also weekly mileage and number of sessions. If you're already fairly fit it's easy to underestimate the strain running can put on your joints and ligaments, and that could lead you to do too much too soon and end up injured. Build up slowly and prioritise recovery.

Outside of that I think it's important not to overthink too much. It's easy to fall down the rabbit hole of stressing about minor differences in shoes, or tracking your heart rate zones, or energy gels, or any number of other things. If you're generally in good health, just run and enjoy it, and the rest will come later. Have fun!