Hi Guys... I'm Usually doing that for an approximatly 15Km Running & walking distance. First, I correctly make gym & fitness to unstress my muscles & adapt my mental "to a ready and operational state".
This has to be made everydays (morning if possible), around 30 and 60mn. It's interesting to make a general program and some personalized ones able to accentuate your job on different body areas;
This way, the starting day, you don't need to work too hard on your fitness program (just around 15 and 30mn, doing it when correctly breathing and relaxing);
When you start, you can walk during 1km, just managing your speed by speeding it up slowly (you'll be around 10 & 12mn/Km; 6 or 7 km/hr); This target is not yours for sure, but :
I live in Africa, Gabon, Central Africa & Tropical Climate; It's hot, I drink a lot of water and try to manage as best as possible all my body to access my targets. So, you can try to, just once.
After the first km, you can start your run... That is the same in F1 to preheat engines and tyres (Lol...!); Don't speed up to fast... do it progressively (500m as you said and walk the necessary time, afterwhich you start to run again);
Runkeeper will tell you all the necessary informations (distance, etc...) to compare Km after Km your progression. You will see that you will rapidly going under 10 and 9mn/km; more if you can run faster and it really depends on the distance you want to or can walk and run a day, on the time you are able to run a day (you have to fix these targets accurately); Try some tests before starting your Journey to verify all your personal data;
Then, each day and each time you will end your "run", take 10 or 15 mn for breathing and not less than 30mn to unstress all your muscles and joints; leg muscles are very important to avoid cramps.
Drink a lot and try to eat well... !
Friendly Yours, Laurent B.