I said once to my DH when I got off my bike on a steep hill that "I need to stretch out my legs for a bit" as I started walking up. Sounds so much better than something like "I've had it", and it's true, anyway! And really, my legs DO feel rejuvenated after they've been stretched out a bit by walking up a hill!
I used to regularly walk up certain really steep hills. Now I "usually" pedal up those same hills, but sometimes if the ride has been long, I'll "stretch my legs a bit" up one or two if I'm really tired.