Build a big engine and then tune it up for performance. Most people tune what they have - you can’t beat the feeling of seeing yourself getting faster and faster times, but slowing it down and building the engine means that you can kick on to a new level. Adjusting intensity downwards is the easiest way to increase volume. Increasing volume is the surest way to improve race speed.
You see all sorts of flavours on how to approach this (Maffetone, etc.) but I like the simple idea of slowing it down so that you can run for ages at an easy enough pace to hold a conversation (nose-breathing, if you are socially distancing). This may include breaking into a walk until your breathing/heart rate eases. If you finish feeling as though you have hardly made an effort then you’ve got it right. Doing this often enough to be able to crank up your weekly mileage should bring speed improvements without having to do speed work sessions. Finishing these easy sessions with a few sets of strides to improve your running efficiency will probably be all that you need for a while. This kind of effort allows you to back it up with daily and then twice daily runs, as the recovery is easy and the body makes the adaptations needed to both build the engine and avoid the injuries normally associated with higher mileage. For most people this is a many months journey and pace at a “hardly breathing” effort can come down by a few minutes/km in that time. The most famous example is Iron Man legend, Mark Allen, who would start his “patience period” of weight training and slow running, at a speed of 4:05/km for this effort and would expect to improve on that by 4 or 5 seconds a week. He would finish this training block able to run at sub 3:20/km at the same effort. Once your pace at this effort starts to plateau then you can bring in speedwork to push up the upper limit.
The thing that gets in the way of improvement using this approach is ego. Having Strava show that you’ve lost 2 minutes a km off your normal run pace is enough to kick you off this wagon and that has been my failing each time I’ve tried this.
I started down this path again a couple of months back and seeing that my pace had slowed in just a few years from just over 5 minutes/km for 21.1 to 8+ minutes at the same heart rate was a big blow to my ego. A couple of weeks of daily slow running (with walking breaks, if needed) had quickly pulled this down to 6.30ish but then lockdown started and I haven’t committed to daily running using this approach on the treadmill. Until today.
I’ll give it a go later and try to keep moving (run then walk, if needed) for an hour and see what pace that looks like. I wouldn’t be surprised if I don’t complete 8km in that time. If I can accept those sort of paces for a while then I should find the consistency to push that to 10km/hour in a month or so, and closer to 12km/h over the next few months.