from 0 to 25 miles race by June?
Greetings and welcome hannes,
If you've not been riding at all, 25 miles may be too much, it depends on your general fitness level starting out.
But, last year, after a 15 year lag, I started weight training in the winter, (tapered off by March) didn't ride lots on the ground (indoor bike and elipitcal trainer) and by June 21st. I was able to do 30 miles on gentle Vermont hills (not a race) on a mtn bike.
I highly recommend weight training, especially for the legs - squats, leg curls, etc. - it made an amazing difference in my legs.
Keep in touch re: your progress and Good luck!:D
coming out of your shell bike wise
I was often terrified when I first started and still am sometimes.
I suggest just keep pushing a little bit. Try doing one scary thing and then try another after that. that's what I did, and am still doing it!