I keep my race numbers as well, and write time & placing on the back. Which made it particularly annoying when the number disintegrated during the Croydon Ultra!
I've also bought an engraver gadget from Lidl for £7.99, I think it was, so I can engrave the backs of the medals with my name, time etc. (including the name & date of the race, if that wasn't included on the medal).