I just bought the Darkside Ravines Legendary Track but I've never bought these sorts of (buy x to unlock a challenger) items before. Do I HAVE to use it right away or now that I own it, can I unlock the challenger when ever I like?

Thanks.

You can hold on to it and use it whenever.

You can choose to never activate it and sell it to someone else and the activation will work fine for them.

You can also just use it as a wearable if you like.