I would use this idiomatic expression but it might be a bit too formal/literary:

— Autant que faire se peut !

To keep in the colloquial tone used in the English dialog, that might be simplified to:

— Autant que je peux !

— Je peux pas être plus !

or, as suggested by Tensibai:

— Autant que possible !

It’s possible to use plus prêt que jamais which as an answer would often be shortened as follows:

— Tu es prêt ?
— Plus que jamais !

Another very common phrase is “C’est maintenant ou jamais !”.

Another current expression is:

C’est quand tu veux !

Well, it does not fully answer You ready?, but it is very common in French, and it seems coherent with the language register used in your example.

A more litteral translation would be

Je n’ai jamais été aussi prêt

For the added context my expression of choice would be:

Quand faut y aller, faut y aller

It is used to self encourage for something frightening one want to do (sky diving, surf riding a big wave seems to match IMHO). It can also be used as an encouragement against procrastinating a difficult task.

Another variation which may convey this meaning is

À la guerre comme à la guerre but more aimed at something reluctant to do or when you have very limited possibility to do it.

It is used to express this very idea of something one is uncomfortable to do but will do with what is available.
I can’t tell if it really fit without insight on which action ‘You ready ?’ refers to.