Are you still getting out walking at least once a day? Still getting those legs and arms pumping and raising that heart rate? If so, well done, to have kept it up is your first great milestone.
We firmly believe here at nutrition-advice.net that milestones should be small and achievable until you have reached the next level of fitness, then you can raise the stakes a little bit higher with milestones that are more challenging. Like my personal challenge to run 8.2 miles around a reservoir on my 41st birthday in July.
Time yourself today on your walk. Did you get to your mile point quicker? If so, you should be pleased with your progress. The next step is to get that training log underway.

As we've already said, you can record it on anything you find convenient as long as you can refer back easily.
This year I'm using an A5 page a day diary, it doesn't take up too much room and is easy to add to quickly at opportune moments. But the purpose made logs are great, and usually full of psychological pickups, hints and tips.

But whatever you choose make sure it's easy to add to daily. In a way a bit like your own private blog.
Caution: As with all exercise, if you haven't exercised for some time, or if you have any medical conditions, you should see your physician first for a checkup. Tell them what you are planning to do so they can advise based on any medical history you might have.