100 miles for someone who's not ridden far before is no laughing matter.100miles is easy.
even when I'm totally unfit and didn't cycle for years I went from 0 > 20 miles in about 6 rides with 2 days rest between, with the first ride being about 2 miles and done in
once you can do 20 miles, 30 miles is just as easy or 40.
obviously at a leisurely pace of like 15mph average
You'll want to be putting in as many hours on the bike as you can in what little time you have left.