I suspect that the same advice we give most people new to distance events makes sense. Approach it gradually. If you will forgive a bad pun, this is a marathon, not a sprint. You have time to develop.
You will want to work on both speed and endurance. Of the two, endurance is more important. Five miles of walking a day will bring with it a good base, if you are comfortably there, choose two or possibly three days to go longer. Typically the longest day is either Saturday or Sunday, only because most people have more time on the weekends.
Gradually increase the length of your weekend training until you are covering about 80% or more of your goal distance for that longest session. Every fourth week or so, perhaps every third week, have a down week to recover.
The medium long event mid-week will trend up as well but as a practical matter, anything beyond a couple of hours is likely too time-intensive.
Also, spend time doing organized events at the 5K and 10K level before you decide to commit to a half marathon. Do you like walking on an organized course because you have a choice? If this is just for fitness, you can do the distance by yourself without time constraints.
Time is the other concern. Most races have time limits. Find out what your goal race is, and be certain to practice moving at a fast enough pace to get it done. That's not necessarily a bad thing since the training response of the body is greater if the effort is greater.
Finally, be cautious. Doing anything repetitiously can stress body parts, and you are unlikely to finish and be satisfied, if you are injured before you begin.
Post a reply on the Bulletin Board