Unfortunately, most of the responses here are incorrect.
Q: Assuming hero took fatigue equal to his stamina. Can hero recover fatigue from surge and use it on another skill within the same attack?
A: Yes, a hero can recover fatigue with a surge and use that fatigue within the same attack if he has an applicable skill. (1)
For details in the rule book: Spend Surges (Combat step 4) comes BEFORE Deal Damage (Combat step 5). The wording of "unused" in the paragraph can be a bit confusing, but based on FFGs response, you can basically ignore it as the meaning doesn't change if you remove it. (Nowhere does it say that this use of a surge is resolved after the attack, that's just where most people tend to think of it since it wasn't used otherwise.) ALL surges are spent before dealing damage. You add range, conditions, hearts, regaining fatigue, etc. all in Step 4, in any order you choose.
Now, to preempt the "You're wrong. Conditions happen after" (because I've heard it before), you may be interpreting what gets the condition when incorrectly. The ATTACK would gain the condition during the surge phase (just as an attack that always has it gains it when the attack is declared). During the Deal Damage phase, if any wounds were received, then the conditions from the attack are applied to the figure, but the ATTACK gained the condition earlier in the combat phase and may or may not apply it to the figure based on the defense roll.
While you have fatigue equal to your stamina you cannot voluntarily spend fatigue.
While you have fatigue equal to your stamina and you would suffer any amount of fatigue, suffer health instead.
Unfortunately, you have misquoted the rulebook to suit your own interpretation. (Though it may be possible that you are reading a translated copy that is different than the English version.) You've emphasized "spend" in reference to fatigue to make some kind of difference. The rulebook only mentions "spending" for movement points, surges, experience points, and gold. It always mentions "suffering" when referencing damage and fatigue. (There may be some cards that do otherwise, but my guess is that these would be unintentional.)
The important part of the rulebook is this (parentheses mine based on clarifications from FFG):
"When using skills (this also applies to any other ability) or moving (this currently only applies to suffering fatigue to gain movement points, and doesn't include moving into or out of effects such as Freezing and Web), a hero may only suffer fatigue up to an amount equal to his Stamina. If any other game effect forces a hero to suffer fatigue in excess of his Stamina, he instead suffers damage equal to the excess fatigue that would have been suffered."
So no, you can not suffer health to use Runic Knowledge.
As a side note, I may end up questioning the movement clarification, but until we get an official response, this is what has been passed down from on high.