Take the sex part as it comes (pardon the pun). It's also about what YOU want. Rather than wanking all day, save it up and screw her as many times as you can. You don't have to fark her once and fall asleep. By being fresh you can fark her all night.
To last longer try going down on her and get excited by watching her excited, change positions and occassionally pulling out can help as can encouraging her to grind away on top of you rather than thrusting. If you really can't avoid coming quickly don't hesitate and make no excuses. You will be good to go in a few minutes. Or get her to give you a BJ, spray it all over her mouth and face, rest and then fark her properly. Don't think about it too much.
Dating girls at work is fine unless your workplace has specific regulations against it. A guy at my work has slept with 5 girls in the office and other girls are MORE interested in him as a result, but he does his moves outside of work. That would be the only thing I suggest ~ meet and attract girls at work but do your physical stuff elsewhere, or at least until you have some trust up if that's what you both really want. Everything for him was a result of 'after works drinks'.
There are several women I hesitated in striking at work when I was younger because of the office dynamic and looking back i should have just gone for it. You just have to be prepared for what happens when you fizzle out but part of being a good pick up artist also means being good at managing, ending or transitioning relationships.