@sarahale get the Marathon Originals Greenguard like
@mjr suggested.
pump them up at 65 (because you're light, I have mine at 75
) you'll be flying without getting punctures, plus those tyres will last you 5,000 miles easy.
I have been through lots of Schwalbe tyres (not because they wear out fast, just because ...) the M+ are the worst for making you feel like you're cycling through treacle.
@Yellow Saddle is spot on.
Make sure you ride the new tyres for a good few hundred miles before the sportive, as all Marathons are really hard to get off/on when new, it gets easier when they have been ridden a while.
Truly, I doubt you'll get a puncture on a Marathon Original, I have ridden with them on a forest MB track by mistake, on the flinty canal path, on the commute strewn with glass without deflation.