[Matteinokun]

机上の空論　kijyou no kuuron = an armchair [impraticable] theory
絵に描いた餅　e ni kaita mochi = it's a pie in the sky
畳の上の水練 tatami no ue no suiren = pratice swimming on the tatami
言行不一致　genkou fuicchi = incoherence between what you say and what you do
言うは易く行うは難し iu wa yasuku okonau wa katashi = easy to say, difficult to do

but there are lot of similar proverbs:
for exemple....

口では大阪の城も建つ kuchi dewa Oosaka no shiro mo tatsu = you can build Osaka castle by words
鞍掛け馬の稽古 kurakake uma no keiko = pratice horse-riding on a wooden horse