For this blues rock exercise you only actually need 1 finger!
You will be playing 12 bar blues in no time! You could however use your third finger for fret 4 and little finger for fret 5 if you want to develop those fingers and play at an even faster tempo.
The blues rock exercise is based around a 12 bar blues pattern in E blues, and the chords have been replaced by power chords. This is fairly common practice for some blues riffs, especially as it deviates from the basic power chord by moving up to fret 4 and 5 – you really get the blues feel by doing this. The great thing about this, is that the same fret pattern of 2 4 5 fits for each of the E5, A5 and D5 chords, so it’s quite an easy pattern to get to grips with, while at the same time you are really getting somewhere on the guitar and making something that sounds musical. There is a lot of scope for variations with these fret patterns – this is just one example.
Work hard on repeating and practicing this blues exercise so that you can play it fairly fast eventually.