One thing that i have found very helpful is using your mana for your fast recruit. the benifits of doing this is much greater than using it on you fast build.
Say you want to build 8000 troops(bowman) and it says it will take 180+ hours. That seams like a long time to wait but everytime you use your mana on fast recruit it knocks that time down by 30%... which ultimately means you can have those troops today if you want them bad enough.
where if you use it on fast build it only knocks off one hour everytime you use your mana on it.
To sum this up....DONT waist mana on Fast Build!! its up to you