Regarding training ... ride regularly, and build your distance slowly and steadily.
For example, you might do longer rides on weekends, plus evenings rides or commuting 3-4 days a week.
-- Keep your weekday rides for recovery rides (relaxed rides to the park in the evening, slow rides to and from work), for speed/strength work (intervals, hill climbing repeats, etc.), and for general fitness rides.
-- On the weekends, gradually increase your distance.
A recommendation is to increase your distance 10% per week. That might work for you, or it might be a little bit slow.
If you're comfortable with 20 km, increasing your distance to 22 km next week might be too easy ... you might go to 25 km or 30 km instead. Then see how you feel the following week. If you went for 30 km and struggled with it a little bit, maybe aim for 33 km or 35 km. If you struggled with it a lot, try it again. But if 30 km felt really good, you might try 40 km.
Listen to your body, pay attention to your fueling, and don't push the distance too much too fast ... but do push a little bit.
In the example of BNT above, his longest ride so far is 46 miles. If he has done that distance relatively recently, he could go out and do a 50 or 55 mile ride next weekend. If that goes well, he could go out and ride 60 miles or maybe a little bit more the following weekend. If he gradually increases his distance like that, he could be up to 80 or 90 miles by June, and the 100 mile ride won't be intimidating at all.
