Problem is as a newer runner I couldn't handle anything faster than Z2 running when I started out with 20km/week.
I tried doing v02 max workouts/speedwork for a few weeks then and I felt the after those weeks my body gonna get injured. The limitation wasn't my lungs or heart, it was my muscles and my bones lol. No matter how much strength training I did then (3 times/ week) there was no change. I had to cutback on mileage almost every week lol.
Bo bian , need to progress slowly. It's this part that needs the most discipline. It goes the same for other sports as well, gym stuff bodybuilding, powerlifting, calisthenics. If not sure get injured and kena vicious cycle.
Actually the strength training and stuff like plyo would work, only issue is that the tendons (muscle to bone) and ligaments (bone to bone), fascia and also the bones all increase in strength in the range of few months and possibly for middle age folks 1 year +. Many dont have that kind if patience and discipline to progressive load via prehab.
Also need proper nutrition, which we seriously dont get much from hawker food, i think one fifthj of our Mg intake our hawker food also cannot hit. Dont mention other stuff like Ca, D3, protein intake, sleep recovery etc.
Some folks also might start off from having osteopenia already for eg (a bone minteral density scan costing ~ 160 dollars can tell you just that), no way just 6-12 months can get to a strong enough stage that can handle even a fast 5k. The prevalence of this in SG is pretty high, something like just shy of 60% for middle age folks.
Dont get metatarsal pain after just a few weeks can go arm chio already.
One way i like to see things - it's not overtraining (that happens not so commonly), it's more about under-recovery.