Indeed, the word Tour is feminine. So, it's correct to say something like Die Tour war lang. The die would be the word's article.
It's valid to use die in this case as person B. However, this means one very specific bike tour. Using die would be
Yes, I did this tour
in English. But person A asks about bike tours in general (which is also underlined by the schon mal).
Person A in English:
Have you ever made a bike tour?
The person B in your example uses das. Person B refers to the action of doing a bike tour in general, not one specific tour. Person B could also answer:
Ja, ich habe schon [mal] eine Radtour gemacht.
Normally, one inserts mal almost always, especially in this construction.
The point here is that it's undefined which tour is meant. The article eine is the point. If person B said
Ja, ich habe schon [mal] die(se) Radtour gemacht.
then it would be clear. However, the das refers to the "generic action" of doing a bike tour (something like eine Radtour machen).
The das of person B in your question is a demonstrative pronoun (Demonstrativpronomen).
Edit: Thanks for clarification @amadeusamadeus; see the answer to "das geht leider nicht" - which noun is the "das" here an Artikel for?.