hi there & welcome!
a lot will depend upon a variety of factors: your age, how much you weigh, whether you have any physical impairments or complications, if you've ever exercised before and your general state of health and fitness.
if your doctor is fine with you beginning an exercise program (and i'm sure they would be - any medical complications would require special advice and guidance), then don't worry about how far you are walking at the start.
15 minutes every day is a really good starting base especially if you haven't really exercised before or for a long time. use stuff from a book, such as you describe above, as a guideline. the pace and hence the outcome of how many kms will be different for everyone. i'm really short and don't have a big stride so the same time at the same pace would still mean different lengths completed.
what you want to aim for is raising your heartrate - and the simplest way to gague that is to be walking at a pace where you get the good old "huffy puffy" - you can still talk ok but you are a bit, well, huffy puffy! if you have a heartrate monitor, great - but it's not essential.
once you have been able to comfortably and easily walk for 15 minutes (and when you realise you're doing the same walk in 10 minutes or less - and need to extend your route!) you can begin thinking about taking it to 20 or 30 minutes each day.
just on another tangent - i can't recommend strongly enough the benefits of meditation for high blood pressure. even 5 minutes every day will have an impact on your blood pressure. meditation doesn't have to be about finding your nearest buddhist temple and ripping out days of OMMMM but about stopping for a short period and just breathing. gentle, natural, deep slow breathing. if you can shut the door on the world for a few minutes, get comfy, close your eyes and just breathe, you will notice amazing benefits to your entire being. i get my doc to check my blood pressure, and then i meditate for a minute or two in the room and she takes it again - she's the biggest convert of all - there is a significant and measurable drop in blood pressure thanks to meditation. just some food for thought!
let us know how you go!