It's also implied in the films, considering how Gandalf The White shit himself.
Just happens he can be surprised by a torch wielding ranger when he's focusing intently on getting the One Ring off a Hobbit. His weakness is PIS, more-so than fire, magic-blade wielding Hobbits coming from the rear and vaginas.
There are counter arguments to Gandalf fighting for days on end, straight.
For one, he is played by Ian McKellen and we know Ian is gay. So there goes any argument that Gandalf fought the Balrog for several days straight.
For another, we do not see what happened at all during those days. Gandalf could have taken a break/nap and hidden himself to get some rest. He could have feasted on Orc flesh while sleeping and hiding. Then he would jump out from behind a corner and scare the Balrog until the Balrog pooped some soot.
The Balrog is slow so Gandalf could have just outrun him on many occasions.
Lastly, we know Gandalf is a pedo because he loves himself some little hobbits. He could have easily been distracted by bates.
So it is not as cut and dry as Gandalf fighting the Balrog for several days straight.
Here's what's sad: I am serious about everything in my post.