there are mitochondrial and other important benefits from "LSR's" just as there are different kinds of physiological benefits from other types of session eg hill reps.
to just run a long way, very slowly is a little naïve....just saying! most exercise will help increase fitness. Marathon training will help your 100m time; 100m training will help your marathon time...just maybe there is a better way by doing the right type of training for your level of fitness; amount of available time and desired distance.
you've got some good advice above. lots of +1's IMHO. I reckon what you do is good BUT you are used to it and as per the earlier comment I think change will be good. although, again, what you are doing in itself is ok.
There MIGHT be a place for your beloved 100m. Use them as strides at the start of each session.
if you are looking to gain time over the coming couple of months I doubt the additional long slow run will make a jot of difference. f you are going to do a long 'slow' run make sure you know what 'slow' actually is.
hi Sketty - I think your 5k specific plan probably wasn't at fault. I suspect mine would have made only some difference at the margins (other things being equal).
But on to the meat of what you say: with 3 runs per week you CAN CERTAINLY improve quite a lot. It sounds like you are following a sprint triathlon plan of some sorts if all your sessions (presumably 9-12 per week) involve at least 30 mins of 'trying' then you simply must be getting better/'fitter' IF your plan allows sufficient recovery and speed (diet etc). If you did your 22:55 'flat out' BUT fatigued then simply by tapering properly you could knock more than 1 minute off.
IMHO it's not all about miles or even speedy miles. There's a lot more to getting the benefit from the miles or miles per hour than just what you do on the ground.
look on the High5 website...they have some very good guides for nutrition at all distances. Funnily enough they recommend their own products but good nevertheless ( I don't use High5). What they recommend seems like too much but try it.