While duelling that close to lava, I would suspect that they would get dangerously hot and dehydrated, and possibly burned. Did they use the Force to protect themselves in some way, for example lower their body temperatures and cool the air around them while duelling?
The DVD commentary for Episode III (with producer Rick McCallum) explains this in more detail. In short, the blue haze you see is not only a repulsor field but it also creates a form of cooling for the Jedi:
Rick McCallum : This blue light around the bottom of the light and the platforms was added later on, in Production. George felt that at one point, that we needed to feel that there was some cooling effect, some kind of shield...a reason why our actors aren't burning up being around this intense heat so Roger Guyett and his team added these shields, this shield generator into this shots as another pass after they'd completed a lot of these shots...to help tell that story of the cooling.
There are many planets in the EU that are unrealistic. One of them is Mustafar. That much volcanic activity would not only raise the temperature of the air but fill it with toxins so as to make the air poisonous to breathe. The land was also shown as basically stable. But the forces required to maintain heat so as to keep the rivers of lava flowing molten on the surface, would require the entire landscape to be tortured by tidal forces, resulting in a constantly shifting landscape that is basically a flat shelf of rock floating on a molten see just a few dosen yards at its thickest.
The Star Wars EU requires some suspension of disbelief. One of them is that there is a planet that has molten lava flowing all over it surface is small streams that does not destabilize the crust, create excessive heat, or completely poison the air.
As Chad answered, there are a lot of things which are considered in our world unreal, like the Jedi powers themselves. We see that Jedi can do a lot by using the force and I cannot see any reason why it should be in this case otherwise.
For example, in the Episode 1, in the very beginning, we can see that they are able to hold their breath in a poisonous environment longer that normal human being. So they definitely posses the ability to survive in hostile environment.
You can also see that Anakin, after the combat with Obi-Wan, caught on fire. My understanding "why not before(?)" is: because his ability to use the force diminished.
So my answer is YES, they used the force to survive under the hostile condition, like they do all the time.