Alexandra. This is the simple answer to your original post. Think of yourself as a tube of toothpaste (stay with it). As a runner in your late 60's aiming to run 3 hours for a half, the level of toothpaste (or performance potential) is massive. Now if you push up the volume and or intensity of your training you are going to get results, or using my analogy, you will squeeze some of the toothpaste out regardless of where you squeeze from. Now if you were at a higher more serious level and looking at improving to something like 1.40 for someone of your age, you would have to think about a training approach that would maximise training potential and thus maximise the amount of toothpaste you can get from the tube (i.e by rolling from the bottom of the tube).

In other words by raising volume or intensity or both you will get better. You really just need to consider avoiding injury and making sure you are used to time on your feet pre event.