I went mountaineering the other day for the first time, and it was harder than I thought. My leg muscles are quite sore now.
I went hiking in the mountains a few days ago for the first time, and it was more strenuous than I thought it would be. Now my legs have been killing me for the last couple of days.
この場合、次のような言い方ができますよ。
ーI went mountaineering the other day for the first time, and it was harder than I thought. My leg muscles are quite sore now.
「この前初めて登山に出かけたら、思ったより大変だった。今、足の筋肉がとても痛いです」
to go mountaineering で「登山に行く」
sore で「痛い」
ーI went hiking in the mountains a few days ago for the first time, and it was more strenuous than I thought it would be. Now my legs have been killing me for the last couple of days.
「数日前初めて登山に出かけて、思っていたより骨の折れるものだった。ここ二日足が痛くて死にそうです」
strenuous「骨の折れる・激しい」
my XX is killing me で「XXが痛くて死にそう・ものすごく痛い」という意味です。
ご参考まで！
カナダ 
