what I do is dependent upon the horse's fitness level from the get go, if they are somewhat fit its different than a pasture potato.
I start with 10 minutes walking, 10 minutes trotting, 10 minutes walking
this branches into slow trot, fast trot, slow walk, fast walk, etc as the horse progresses fitness wise.
gradually lengthening the time. Say the event I want to do it 6 kilometers, I will condition to 7 or 8 not just stop at what the event length is.
One day is dressage day, 1 day is trails, etc and sometimes I mix it up and do mix of things such as if we've been working on circles and transitions and doing well then I'll end that for the day and go out on a short trail or do something fun. Its just a matter of the horse. You can't do hazards all the time, the better your dressage becomes the better everything else will be.