Run
If there was one thing a companion learned the quickest whilst travelling with the Doctor it was that you needed to be fit because there was one thing that you could guarantee to do after you step out of the doors of the TARDIS.
You had to run.
They learned this quickly enough because most of the time, the first time a person meets the Doctor is in a life threatening situation... Most likely with an alien threat (always with an alien threat, actually)
The Doctor reflected on this as she sat in the doorway of the TARDIS, the doors were wide open and her legs were dangling outside as she overlooked the planet Earth and wondering what every single one of her past companions was doing. Did they know that she was looking out for them all? Because she remembers them all. In every new companion she sees bits and pieces of the ones she has left behind.
"What are you doing?" And then there was that one. The unpredictable, moody and equal man she had the unfortunate pleasure to call a companion. The Master sat next to her and leans back on his hands, looking down at the Earth. "Not still moping, are you?" He asks and turns to look at the profile of his friend - because they were friends now - before looking at the ring on his finger, the one that the Doctor uses when he gets out of control and she can't physically stop him. She had given him the ring after the Year That Never Was and whilst he hated it - and her - It made sense. They were the last of the Time Lords, he didn't want any of them to die.
"I don't mope," The Doctor finally replies and turns to look at him. The Master ignored the obvious fact that she had previously been crying and instead got straight to the issue.
"She wouldn't want you to. Jenny was a soldier, like her mother." He was prepared for the slap on his arm and refrained from rubbing it so as not to make her happy. But Rassilon that hurt! For such a scrawny thing she was strong!
"I am not a soldier! She was nothing like me," She scowls and crosses her arms, bringing her knees to her chest and looking at the Earth once more.
"She was exactly like you," The Master says, standing up and walking away. "She enjoyed all the damn running we have to do!" And the Master started said running as the agile Doctor jumped to her feet and started chasing after the only companion who wasn't exactly a companion. He was her equal in age, intelligence and species.
That, and the fact he never did learn that travelling with the Doctor was just a fancy way of saying 'constantly running for your life'.