I found it very easy to play when I broke it up and played it slower
I looked at it as a "triplet+1" so D U D + U as it has been said. The last up actually makes sense since it prepares you for the next downstroke
a good exercise that got me to finally get this down (and not to mention good for stamina) is to do that picking pattern for a minute, at a slow termpo, make sure to get the rhythm of it
then increase speed or increase time played
I got it to a minute first, then did it 2, then 3 minutes
then increased speed and repeated the cycle at a lower time, building up
Afterwards it isnt such a challenge to play it for extended periods of time
feel the burrrrrrrn!!!