Trying to maintain core and upper body definition but not gain an extra muscle mass.
I sincerely hope you are NOT doing these exercises "to maintain definition."
because, that would be a silly thing to do.
but you may well need to do upper body exercises as part of your core routine.
whether you need to do them depends on you, as an individual. training is specific. you do specific training to make specific adaptations to make you better suited to meet your performance goals. if your core is made of sponge cake then you need to firm it up and make it work for you, and this has nothing to do with definition or muscle mass.
your core is, as its name implies, at the core of your posture control. it determines the extent of your movement vocabulary. it is fundamental to the type of runner you are.
and, incidentally, you haven't been doing weightlifting.
cheers.