Bale has barely featured for Los Blancos, having played just 77 minutes of action since the November international break, but the former Tottenham star said he is capable of playing as much as he is needed for his nation.
Initially there were fears Bale wouldn't be available after missing El Clasico in which his side were humiliated at home by Barca.
Following the match, his manager, Carlo Ancelotti said: "He wasn't feeling well yesterday after training."
But the 32-year-old played down fears and is ready to do his best in front of a sell-out crowd at the Cardiff City Stadium as Wales take on Austria in their 2022 World Cup play-off.
"It's normal to have some niggles now and again. I've been training for the last two and a half months," said Bale. "I got some minutes maybe three weeks ago or so.
"I'm in very good shape, I'm in a lot better shape than I was in the last camp where I'd been out for about three months.
"I'm in good shape, I've been training now the last couple of days and ready to play whatever is necessary tomorrow and we'll give it everything we can."
Transfer expert Fabrizio Romano said the 32-year-old will leave the Bernabeu on the expiration of his contract in June.
On his contract situation at Real Madrid, Bale added: "I haven't thought about that at all, since we knew we were going to be in this situation, my sole focus has been on this.
"I know that thinking of the future can cause doubts in your mind so I've not been thinking about that, I've just been focusing on this game coming up.
"I know that thinking of the future can cause doubts in your mind so I've not been thinking about that, I've just been focusing on this game coming up.
"Whatever happens in the future I'll decide in the future."