I'm going to miss my bus
Why are you going to miss your bus?
Because I'd like to go for a walk and I need some time to myself to think.
It's understandable to sometimes need to take a walk to think, but you shouldn't do so at the risk of missing your bus. It's important to be on time places, otherwise we'll be viewed as unreliable. Try leaving a little bit earlier to take your walk so that you can get to your bus on time.
I can't leave early and it's not a big deal to miss my bus.
You might not think it is a big deal but it could be to other people. You should always try to be on time because it is responsible. If you want to walk, you should try leaving earlier so that way you can't miss the bus.