It's hard for me to walk for a long time because I don't have much stamina.
I don't have very good endurance which makes it hard for me to walk for a long time.
どちらも「体力があまりないので、長時間歩くのが辛い。」という意味です。
1) It's hard for me to walk for a long time because I don't have much stamina.
2) I don't have very good endurance which makes it hard for me to walk for a long time.
「体力」は stamina や endurance を使って言えるので
「体力がない」で
don't have much stamina
don't have very good endurance
などと言えます。
「長時間歩く」は walk for a long time と言えます。
ご参考になれば幸いです！
日本 
