No. You can use it just 1 day out of the 10 days if you want. Say you buy it September 1st...it'll expire September 10th and you'll lose every MB that you didn't use. If you use all 100MB on one day or before the expired date, then you'd have to buy another 10MB if you want to continue using Broadband2Go.
Virgin Mobile answered your question in the FAQs:
• If I'm currently on a Virgin Mobile BroadBand2Go $10/$20/$50 plan, will I be able to keep my plan?
Yes. If you bought a $10/$20/$50 plan before 7/4/12, you will be able to keep your plan at this time, unless you switch to another plan or until the plan is discontinued. However, if you are on one of these plans, you will not be able to enjoy 4G speeds, even if you have purchased a 3G/4G device. http://www.virginmobileusa.com/mobile-broadband-plans/broadband-2-go/overview/#faqs
• Yes, if you're currently on a Virgin Mobile BroadBand2Go $10 plan the 10 days are consecutive.
• The Broadband2Go $10 plan has expired and is no longer available to new customers.