The bad news is that 100 miles on any bike is a quantum leap from where you are now. The good news is that 100 miles on an exercise bike, if you can deal with the boredom, is much easier than on the road -- no wind resistance, no environmental hazards, bad roads, etc.
First off, visit your doctor and get cleared for starting an intensive fitness program. Let him know exactly what your plan to train to do.
Once you have the medical clearance, try to set aside one day a week when you do a big ride (maximum distance). That one big ride each week (which currently may only be 5 miles) is the one that will eventually build to your 100 mile goal.
Since your immediate goal is to do this on an exercise bike, your training can involve pure endurance, i.e. you won't have to worry about speed or climbing skills.
Here is a seven day plan that will get you to your goal by increasing your training rides by 10% each week. You can do these either on your road bike or on an exercise bike, as desired.
Day 1: 5 very easy miles, increased by 10% each week to a max. of 15 very easy miles.
Day 2: 5 miles, increased by 10% each week to a max. of 15 miles.
Day 3: 5 miles, increased by 10% each week to a max. of 25 miles.
Day 4: Take the day off, or ride an very easy 5-10 miles.
Day 5: 5 miles, increased by 10% each week to a max. of 25 miles.
Day 6: 5 miles, increased by 10% each week to a max. of 10 miles.
Day 7: 5 miles, increased by 10% each week to a max. of 75 miles.
The 10% increase each week is a goal, not a requirement. If you're not comfortable with a new increased distance, drop back for a week or two until you are ready to progress to the next level. Listen to your body, and don't be afraid to take additional days off, or substitute easy rides as needed.
Good luck!
