The idea that a long run needs to be run slowly is misguided and as you say will only tend to prepare you to run slowly.
Don't slow yourself down
run at a pace that is comfortable, feels like you've tried but doesn't leave you exhausted for next time.
To improve your speed, try hill bounding. maybe 100m on a moderate gradient. at the top run 100m across, then 100m down, then 100 back to your start point
You don't have to race up the hill just strong steady controlled strides that get you to the top in reasonable shape to jog on.