I don't know your physical fitness history, so just a couple general tips.

Especially if you're starting from a condition with not much running, particularly running with a load on your back, your muscles and cardiovascular system will typically improve faster than your bones and tendons/ligaments, particularly in the earlier phase of training.

So, just be aware that injuries like stress fractures or popping an Achilles could be more of an issue now than later on in your training, after your skeleton and connective tissues have had a chance to toughen up more. In other words, listen to your body and don't just look at a decreasing time as your indication to add mileage or push the pace harder. There's nothing more demoralizing than getting knocked out early in your training by some injury that takes weeks or even months to heal. As we get older, we just don't heal as fast as we once did. Once your body is more accustomed to hard training, you can be more aggressive in improving your performance.

Also, don't forget that your improvement will not be linear so don't be a slave to some gradually increasing workload. You'll go through phases where you improve quickly, improve slowly, or hit a plateau and not improve at all or even slow down. Just roll with it, maybe vary the workout in some way when you hit a plateau, and eventually you'll be on the upslope again. Good luck!