This. Also, don't just rely on Google Maps estimates - ask on here.
For instance, I can get a train to London Bridge in 15-20 minutes, then hop a Northern Line train to work in 5 minutes. In theory.
In practice, the Northern Line is so manic at that time of morning that even if I could get on, I wouldn't want to. So it's a 25 minute bus ride or a walk of the same length.