It sounds like the clutch master cylinder
(and/or slave cylinder) May be about to fail.
Firstly you need to check if there is any moisture on the slave cylinder
- around the boot area.
Its in the engine bay bolted to the gearbox bell-housing next to the exhaust manifold.
You really need to get under the car to see it.
Either get the rear onto jack stands or ramps.
If it isn't wet, the problem is more likely to be with the master cylinder.
There isn't necessarily going to be any leakage from the unit as the internal seals fail allowing fluid to bypass when you press the pedal.
Some members describe the pedal being
'stuck to the floor' when the master cylinder fails.
There is also reports of the clutch fluid breaking down due to the heat of the exhaust.
This would be my first suggestion if you can't see anything obviously wrong with either cylinders.
Try bleeding a bit of fluid off and see if this improves things.
When I did this to mine the fluid drained off was black and very watery.
A word of caution.
DO NOT allow any air to get into the system when you bleed it.
Make sure the res is fully topped up and only do half a pedal stroke at a time.
Good luck