I felt really light as if I am only one stone, but I couldn't find the energy to accelerate a bit faster even though i was running confortably - that was 12 hrs after my breakfast.

Is it advisable to run on empty stomach?

Cheers

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Different people have different needs... as long as you have a reasonable diet and don't starve yourself/regularly skip meals then your body should be fine!

I try to eat 2 hours before a run and/or asap afterwards, if I run on empty I can run okay but just lack energy/strength. If I eat too close to running I tend to feel unsettled throughout the run

Pretty much what Richard said.
Running on an empty stomach shouldn't be wrong in itself. The problem cmoes if you lack energy, and therefore isn't able to run as well

I find that if I go for a run some 10 hours + after the last meal I tend to get back pain after some 50 minutes/ 1 hour.I thought this wasn't something normal but a friend of mine has had the same complaint recently when training for his first marathon.

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I would suggest to eat 1 to 5 slices of toast bred an hour before going running (depending on the kind of run you are planning) so you get some energy in you. Dark bread with seeds is recommended. Granary bred has some protein in it.