"I thought I could easily ride for 20minutes but after 5 minutes I ran out of oxygen."
"I thought I would have no problem riding for 20 minutes but I was struggling to breath after 5 minutes."
"I thought I could easily ride for 20minutes but after 5 minutes I ran out of oxygen."
"I thought ~ but" 「〜と思ってたけど」
"I could easily ~" 「余裕で〜」
"ride for 20 minutes" 「20分位漕げる」
"after 5 minutes" 「5分で」
"I ran out of oxygen" 「酸欠状態だった」
"I thought I would have no problem riding for 20 minutes but I was struggling to breath after 5 minutes."
"I thought I would have no problem ~" 「〜で余裕と思っていた」
"I was struggling to breath" 「酸欠状態だった」"gasping for air" も言えます。