It really depends on your goals. I know some people just train to finish brevets. That's fine, you don't have to do much training then. If you want to train scientifically, there are plenty of people that will help you do that. In my mind, there is very little difference in training to race a bicycle and training for randonneuring at a high level of performance. When I was racing, it wasn't that big of a deal for me to ride 100 miles. The thing that riding long rides will help you do is sort out your equipment and fit issues. You could do a similar workout schedule to what you have outlined: 2x20 minute intervals separated by 10 minute cool down twice a week, a 60+ mile long ride once a week, gym 3 days a week, and one rest day.