...I shall ride 25 Km in the cyclo park first...
Another 2p. As a training goal you could see if you can eventually build up do 25km in 1hour. Not that I've done an Audax at an average speed of 25km/h before, but over 25km - if have reasonable potential fitness, may be a challenge to try. Training on the same track could be 'boring' though, so after a while perhaps take it to a decent handful of diff routes...
Varying the routes isn't just a nice thing to do to make training less boring - as per previous post, experience of road riding is essential.
There's too much well-meant-but-misguided positivity in this thread, and focusing on daft stuff like inner tubes instead of what actually matters. As an organiser, I wouldn't be happy about accepting an entry from the OP - for their own safety.
Apologies for any potential bunkum spouted.
I was kind of assuming since the OP was thinking about joining a cycling club & doing Audax, that they had resonable road riding experience to build on. I went through several years of hardly riding at all, but prior to that had had experience of commuting in London / cycle tours etc, but anyway...
[Maybe if OP doesn't have much previous road cycling experience, as other have said, 'all things in good time' build it up slowly, safely, concentrate on building confidence / road skills first, perhaps riding on familiar quieter road routes, at quieter times of day, gradually increasing distances; go out with any experienced cycling friends...etc]