What activity should I use when running with a beginner?

I am helping my girlfriend start runing. She is doing a Couch to 5K plan at the moment and being winter here in the UK she wants me to run with her when its dark etc. 

Obviously being a beginner she doesn't run too quick yet and stops a few times. What do people recommend I use to track my activity but that won't affect my running statistics, like VO2. Would just a Trail Run suffice?

Thanks for any help.