Wee Curly Dee, the point of the slow LSR is to help you build endurance - you get that slowly and gently and gradually work up to doing it faster. The short runs are essentially recovery runs designed to help your body expel toxins/loosen off again after either shorter fast sessions, or longer endurance sessions. The slow runs will seem ludicrously slow if you've been running at the same pace all the time over the years, but that's exactly what they're meant to be - although I know a lot of us struggle to go quite as slowly as we ought to and sometimes it can be painful to slow down that much, so a wee increase in pace to get comfortable shouldn't be a problem
I think that essentially, the combination of running longer distances, in a way that isn't hugely stressful for your body, combined with shorter speed sessions challenges your body more effectively than plodding along in the same way all the time. The fact that you're finding the speed sessions actually challenging means that you should start to see improvements on the tempo runs, as the pace on these drops, the speed session training allows you to not collapse in a soggy heat.
Perhaps re-run the schedule over 16 weeks to see how it looks as it builds up, or look at a marathon schedule to compare. You'll see that the LSR pace gradually increases, as does the tempo distance/pace & the speed session distance/pace.
There was a bit of debate on here about the smart coach not challenging enough if you wanted to improve your time over a distance you'd already completed, and I can see that. It doesn't push too hard if you're using a 10k time to get a 10k schedule. But if you're using it to increase distance and motivate you, I'd say that it's worth persevering. The majority of your runs should be easier/as easy as you can do already, with the longer runs increasing over what you're already doing & the speed sessions effectively running you into the ground. 
Does that help at all?