Learn to roll! We start you slow on purpose. Don't go too fast on the long notes, or you'll be struggling when you get to the short notes. Focus on accuracy, playing relaxed, and reducing excess motion.
The important thing to notice about rolls is that it is not about the strings you use, but the fingers used that define the picking pattern. Here's Some examples: TITM - Alternating Thumb Roll, TIMTMITM - Forward Backward Roll. Pay attention and memorize the rest, then try that picking pattern by using different, or even a single string!