I recently rode down a mountain that I was unfamiliar with and rode my brakes excessively. One brake lever now when squeezed doesn't pop back into place. do I need to replace the brake cable or would it likely be something mechanical inside the brifter?

V brakes? If the return spring in the brake isn't broken.. dirt in the cable housing perhaps?
test.. take the cable off the brake ,

just like mending a puncture, and removing the wheel
[except for the wheel doesn't have to come out].

and see what pulling the cable directly feels like.

feel the caliper motion too, there may be a need to take the brake off the frame
and clean and re-grease the bushing /boss it rotates around.

then do the same to the other brake It's likely about due

It could very well need a bleeding. It's a disc brake, right? If you feel like you need to bleed them, I just went through a terrible hassle trying to bleed mine and I finally got it and now I feel confident I could help. What type of brakes, and what brand?

It doesn't sound like hydraulic discs with brifters. maybe gunk in the cable housing or the housing has a tight bend.

Rode down a mountain? Is there now a high spot on one of the brake pads that allows it to hang up just under the edge of the rim? If not, I would check the cable for fraying.

You can isolate the three parts of the system by disconnecting the cable from the caliper. Now check the caliper for hanging up by pinching the arms closed with your fingers and then releasing the pressure a little at a time. The arms should maintain the same pressure on your fingers the entire way. If it hangs up for a moment and then returns to full pressure then you're problem in in the caliper or the pads have a lip that is hanging up as suggested above.

If the calipers are good then try pulling the lever, release it and then pull on the far end of the cable. If the cable is still to pull back then it's the brifter arm or the cable. So strip the cable out of the housings and brifter. Re-check the lever to ensure that it's moving well. Study the cable for kinks or frayed strands. Somewhere in one of these three parts of your brake system a problem should become obvious.

The other thought is that if it has been a long time since you last changed out your housings that perhaps the inner liner has worn a groove in the plastic and the cable is being wedged into this groove instead of slipping freely in the usual loose fitting housings. Housings are actually a consumable part just like brake pads and chains.