It depends on lots of factors. What is your current fitness level? What do you consider an average ride for you now? How far do you think you could ride now (on terrain that is similar to the course you want to ride)? Is your goal to just finish it?
Everybody every course is different, but the old rule-of-thumb is to increase by about 10% per week. You have about 8 weeks to go. So if you can ride about 50 miles now and you add 10% each week, you would be at the 100 mile goal near the end of August.
And even if you don't make it to your goal, it's still good! You're out riding your bike!![]()