So, I don’t know your footage and if this could work and actually save you some time. But…
you could always do a rough roto and input this into a luma keyer. Luma keying means that you make a mask from a difference in brightness. That would require your background to be darker (or brighter) than your foreground. this would work for a uniform black (or white) background even without the rough roto.
If you have a cleanplate of your background, meaning an image or image-sequence which does not contain the foreground element you want to isolate, you could use an image difference keyer (PIK node in Natron) for masking out the background. this is further from automatic, usually still done with rather uniform greenscreens. I am guessing that the background does not have to be green for the math to work…but that depends on the implementation.
As you might have guessed, a compositor like Natron has a lot of tools to achieve what you want to achieve, all there to be higher quality and less time consuming than manually adjusted roto-shapes. But they all still require some knowledge and a certain level of foreground-background separation. The better that separation, the better all of this will work (even the stuff that @bazza postet works better with more uniform backgrounds)
Cheers!
p.s.: is that 25min of footage already shot? can you reshoot with a greenscreen? If you can reshoot and you are not proficient with natron…reshooting properly is much simpler than trying to fix horrible source material in post.