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if your 5k and 10k times aren't in line (suggesting an improvement in endurance fitness will help get your 10k time down)
For me it's weight that hinders my endurance fitness and my VDOT. I couldn't get near the 10k time that was suggested by my 5k time and it got even worse with the longer distances.
I used this to help me keep track of my improving fitness as I trained a few years back (attempting to lose weight and get fit enough to survive a marathon). Of course I've put the weight back on and am starting from square one (for the 3rd time now).
I stuck each run in a spreadsheet and had it auto calculate the VDOT score (the formula is easy to find online). I then kept track of the VDOT score for the major distances: 5k, 10k, HM, M. (And differentiating between a 5k 'race' like parkrun and a 5k training loop, or my 10km training loop and an interval session that just happened to be ~10km.)
What I noticed is that as the weight dropped off all the VDOT scores increased but the gap between 5k VDOT and HM VDOT narrowed, as the weight increased so did the gap (even though "fitness" was being maintained or even improved).
I guess that at some weight the VDOT scores for the various distances would settle close together (assuming they came from a race at each distance). No idea whether this weight would be attainable for me but there's only one way to find out. The improving (and narrowing gaps) VDOT scores were definitely motivational.
(What I'm trying to do in the copious free time I don't have any of is automate the graphs and stats stuff so all I have to do is upload the .fit file and my stuff will analyse the .fit file, work out what kind of a run it was (by comparing it to my usual routes) and then tell me how I'm doing. Something like a cross between Strava, Garmin Connect and TrainingPeaks but without any of the Social wank. Swimming and cycling too as I am proto-slvlss.)
Anyway, less talk, more running (once this lurgy is gone from my chest).
Another pace/race calculator I like is McMillan. As well as being pretty close on my own race paces in terms of predictions, I like the feature of having a pace range for different work-outs.
I agree with erring on the side of caution for a novice; if your 5k and 10k times aren't in line (suggesting an improvement in endurance fitness will help get your 10k time down), you could use the calculators to give you a range based on both times, so that you aim to run inside the 10k-based paces but make sure you're not running quicker than 5k-based paces... until your fitness improves.