Carrots! Lots of carrots!!!
Better if you use conical shaped ones, like
this.
Proceed like that: lube one carrot, insert it, fuck yourself with it until edge, repeat.
When your reach 3, your routine slightly changes: lube one carrot, add it to the bunch, edge, spend one minute sitting on carrots to force them to stretch your cunt, repeat
I'll wait for a report on how it went, and what was your maximum by the end of the dare. I may have other ideas for you...