what do you think I should do?
HannahnotAgnes You could switch the one you walked back to your buddy and attempt to evolve during the day in the hopes it may work, but I doubt it as I think it has to be continuous 10km then evolve.
I would only try that if you have plenty of Eevee and Eevee candy (and you don’t mind if that particular Eevee doesn’t end up as Espeon).
Otherwise, there’s nothing you can do - you just have to start again.