Uh, have you ever thought it wouldn't go better if you slowed down and enjoyed the ride? When she is ready to ride faster she will.
Suggestion... do your ride before riding with the girlfriend. Go out and do 30 hard and then come back and cruise with the girlfriend. Or use a mountain bike with wide knobby tires. Better work out for you but hard to go faster.
This really shouldn't be about you but about riding together and doing an activity as a couple. That means compromise. And I guess a tandem would solve your problem (provided she is a trusting girlfriend. Riding as a stroker can sometimes be scary).
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