I have mild leg weakness. i find when i'm tired it seems worse and becomes hard to do activities such as climbing stairs. is this expected? any advice

Yes, with weakness -: Your symptoms will get worse. One needs to determine why do you have weakness in the first place? Se your pcp to be evaluated/investigated ; treated. Any infective condition ( with viral being rather common) will give you generalized weakness; a back issue with pinched nerve will probably give a one-sided weakness; or it could be due to a local soft tissue or bony lesions. Might need an orthopod.