I think both Cait and Sarah are right, but Cait is arguing from a biological point of view and Sarah from a cultural point of view. Certainly from a cultural point of view, certain behaviours are hardwired into the dominant cultural discourse especially once they obtain the status of being myth (i'm talking about Roland Barthes' concept of myth where a social ordering renders a statement of fact as 'natural', therefore making it incontrovertible). Yes, these behaviours and the gendering associated with them are inherently constructed, but within the framework of society, their status as myth means they are viewed as inherently gendered. Obviously with all things cultural these behaviours can and do change (society is fluid after all), the best example i can think of being dressing babies in blue or pink to denote gender, that shifted some time around the late victorian times from pink would be used to denote maleness in a baby, to it denoting femaleness in a baby (I'll try and get a citation for that). So yes, gender roles do exist, they are part of the language of control used by society to order things, even though they are not inherently biological traits.
As to the original question I don't think i overdo things in terms of trying to play a role, though i'm sure others would disagree with me, I largely like the same things i did pre transition and I certainly don't think i'm deliberately playing a role to validate my identity as female.
