You'll be fine, as long as the extension cord cable is of reasonable quality.
No, your pc won't heat more with longer extension cord.
The problem with cords is that the cable has some resistance and there's some power loss in the cable. So the more power what you plug in extension cord consumes, the more the extension cord cable will drop some voltage and heat up (so instead of 110v your pc will "see" 105v or something like that.
These days most power supplies can function with as little as 80v so not much to worry about... and your computer is not really that power hungry, even overclocked... it's just 3-500 watts or a few amps of current (depending if you're in 120v land or 230v) ... losses in the power cable will be around 5-10 watts at most, which can be easily dissipated across 25 meters of cable.