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!