After years of trying and extending distance and time on the bike I've come to a simple conclusion: The best thing to do is be very modest and set up something that you can do each week, week in, week out - that is your base mileage.
And it's better for that mileage to be right than impressive - so if its 25km you can consistently do, then do it. Much better that doing 50km one week, then nothing the next (and having an injury).
The benefit of knowing your base mileage is that when you go beyond it you immediately know you are taking a chance - will your body react right, or is it going to lead to problems? And that's what you need to know in order to maximise your training.
Italian events and European climbs