There's an interesting article published in 2014 on Competitor.com by Jeff Gaudette which addresses the issue of 'Speed verses Aerobic Endurance' — the question of how much short distance speed does one need in order to run a desired time in a longer distance race, be that a 5k or a marathon. John states that 'speed is rarely the limiting factor in how fast you can race, even for a distance as "short" as the 5K.' The limiting factor is aerobic endurance. A runner's 'speed' over 400 metres to 1k is pretty much set genetically. A distance runner's job is to run as close as possible to that speed for the time of the race (which could be as little as 13 minutes for a 5k or longer than 4 hours for a marathon). The 'secret' to fast distance racing is to be aerobically strong enough to hold one's speed for the distance of the race.
John says "there is a limit to how much you can develop your absolute speed. At some point, your body approaches its natural talent point and working to improve speed provides diminishing returns. Luckily, improving your aerobic capacity is virtually limitless." For myself, I still feel like there are big gains to be made in my aerobic capacity, even though I'm now running 80 or more kilometres per week.
So, how fast am I over one kilometre? This is something I haven't tested in a very long time. I think I will though, just to have that information. My guess is under 4:20 (I can run a 4:30 k split in the Parkrun). Now when I was very young (34 or so), I could run a training 1k in 3:09 and ended up racing the 5000m in 17:33 (3:31 per km). My 5k race pace was about 11% slower than my pace for 1k — I wasn't a great 'converter' of my 1k speed into a 5k time. Looking back to those days now, I can say for sure that I hadn't maximised my aerobic ability. A fast runner who was also aerobically strong might be 4 to 5% slower than their 1k speed in a 5k race. That is, if they could run 2:48 'all out' for 1k they could probably hold 2:56 pace for 5k race.
How much speed do I need to run 5k in 22:45? When I was 50 years old I ran 21:29 for the 5000m on the track when I could run 4:00 for 1k in training (about 7% slower for 5k pace than my 1k pace). Extrapolating from this information, presuming I could run 1k in 4:15 and was as aerobically strong as I was in 2008, I could expect to run 5k in 22:44. Now all I need is to be feeling good on a cool, calm day!
Training on the soft grass of Yarralumla Oval on a warm Monday evening