Lower abdomen and lower back pain. Did a lot of yard work this morning. Only a few hours. Am I out of shape at 70 or injured?

It could both. Usually after working hard you have muscle fatigue usually go away after 1-2 days, take some motrin. If it persist and continue the severity you better check it out. Good luck, thank you.
May be not. Lower abdominal and lower back pain could be musculoskeletal related to overuse or an acute event. You may have not been properly hydrated, not properly stretched and may have simply overdone it. "out of shape" is not measured necessarily by having lower back pain the next day.