"If you haven't been keeping up your fitness, when you go to the mountains you'll get tired straight away."
- "If you haven't been keeping up your fitness, when you go to the mountains you'll get tired straight away."
"if you haven't been keeping up your fitness" 「運動不足だったら」"if you haven't been exercising" も使えます。
"when you go to the mountains" 「山に行ったら」
"you'll get tired straight away" 「すぐに疲れる」"you'll tire out straight away." も言えます。
You're probably going to get tired quickly if you go to the mountains because of a lack of exercise.
「運動不足で、山に行ったらきっとすぐに疲れますね」という意味を伝えるフレーズとして、「You're probably going to get tired quickly if you go to the mountains because of a lack of exercise.」があります。
probably: おそらく、たぶん
get tired quickly: すぐに疲れる
lack of exercise: 運動不足
オーストラリア 
アメリカ合衆国 
