There are a number of pacing/training pace calculators on the internet - I like and mostly use Jack Daniels ( American trainer en.wikipedia.org/wiki/Jack_... )
He basically proposes 5 different training paces - long, easy, tempo, interval and repitition (in declining order of pace) . It all sounds very technical and I guess it is - but fairly simple to understand. He gives a brief description of each pace and it's training purpose in his calculator . So for example my training paces ( dependent on a 5K race pace of 31 :00 minutes ( and rounded up a bit for convenient memory) are long 8:10 mins per k, easy 7:45, tempo/threshold 6:30, interval 6:00 and repetition 5:40. As an example , yesterday I did a 10 minute easy warmup run to a track - then did 3x400m repeats ( a bit over 2 minutes each) with a 2 minute rest inbetween each. Then another 10 minute cooldown easy run back home . I will slowly build these short repeats up to around 3 minutes each with a 3 minute rest inbetween until I reach around 6-7 repeats - which will give me a total of 21 minutes workout at 5:40 pace. Just once per week at this pace. he also has other suggestions as to typical once per week tempo paced and interval paced workouts - mostly for a max of 20 minutes plus warmup/cooldown. - plus a longrun each week building up to 90 minutes non-stop. An ongoing programme which has no race distance in mind - but which he says will allow people to participate in 5-10k amateur races.