That will do it

On the Alexis path the MC has been trying, my guy never heard of condoms or pulling out
Running was a side activity for her, not something she did competitively.
Women in particular get hired for looking good, suppousedly encourages dudes to drink more, stay longer at the bar.
But there's always those that take full advantage of things that would go to waste...
And at a bar it's gonna be wings, ribs, fries, burgers...
For Alexis that's not going to happen suddenly, not possible in the timeline between now and the endgame.
Despite the fact that she doesn't exercise much these days, she's very skinny, her dad was a pro basketball player, her mom is super fit

Her family and genetics don't quite suggest that's what she'll become.
Like i said the only way she gets fat is if being dumped puts her into a depression.