I may have done 14 occasionally, but that wouldn't have been a straight 2-a-day scenario... sometimes I'd have a couple of 3-run-days*. But the "norm" would've been twice a day Monday to Friday, with once on Saturday and Sunday. VERY occasionally twice on a Saturday when work dictated.

* These days would be:
1) early morning run.
2) Late afternoon run home from work.
3) Evening club session.

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GONE RUNNING

Hmmm
I want to start feeling that huge improvment that come with two-a-days...
Should I start with 2 5Ks a week (on top of the rest)?

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If you and a friend are in the forest and a bear appears, try to run faster than the bear! But at least run faster than your friend...

Many years after I first employed "2-or-more-run-days" I came across some basic guidelines for the introduction of such an approach:
2 runs on 2 days for 2 weeks, following on with...
2 runs on 3 days for 3 weeks... and so on.
Obviously only you can determine when and where these additions are placed with respect to your other training.
I doubt if there was much structure to the way I started out down this road. I've never really followed any plans at all.
Run hard.
Run often.

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GONE RUNNING

Thank you
But now... going back to your penultimate line...
Ever since I started running people tell me I trained too fast.
I went through 3 months of running withour one single run slower than 4'00''/Km (something like 6'27''/mile).
No reps, no hills. I was just running through the Polish Winter.
And I felt good, felt strong. Never had a chance, during that time, to test myself in a race...
Training for the marathon has taught me to run slower (here I think it's necessary).

But now, going back to the track... What should I do?
Run hard?
Like many good runners, run easy on longer-run days and run very hard on reps/hill days?

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If you and a friend are in the forest and a bear appears, try to run faster than the bear! But at least run faster than your friend...

I'm afraid I rarely trained on the track. It was just a convenience thing with me. All I used to do was run to work and back. Quite often the runs home (5 miles being the shortest route during my peak period) would all be flat-out. I'd literally time-trial every day from mon.-fri... attempting to beat the previous day's time on each run. Track training just didn't "fit in" to my schedule.
I managed a 3-54 1500m/1-57 800m without setting foot on a track in training.

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GONE RUNNING

Ahh, I meant going back to the track as in - I want to go back to competing in track races.
But I also prefer to train outside. Forests being my favourite place.
But I do enjoy some track reps once in a while if running in a group.
It's a bit like a race simulation.

"I managed a 3-54 1500m/1-57 800m without setting foot on a track in training."

Can I borrow your genes, please

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If you and a friend are in the forest and a bear appears, try to run faster than the bear! But at least run faster than your friend...