Hi,
let´s say 26V and 100mA. This gives 2.6W
but you want 12V x 5A = 60W,
You can´t ignore law of physics.
So for continous power you can´t expect much more than 2W. (2.6W reduced to 2W because of expectable loss)
Thus the only way is to work "discontinous".
Store 2W for 1 hour (example) into a battery and you have 2 WattHour of energy in your battery.
If you have a suitable battery than it may deliver 60W for 2Watthour/60W = 1/30 hour = 2 minutes.
.. oh, I forgot to reduce it by the expectable loss, so better expect 1.5minutes.
Klaus