Try something like this which I coached a new gym member this morning. He is overweight and out-of-shape, but he did remarkable well with this routine. Adjust the treadmill speed accordingly, but don't make it too easy.
I had him do a 15 minute
treadmill running workout. We set the incline at 1%.
The 1st two minutes he walked at 4.0 mph. Then he did intervals of 1 minute running and 1 minute walking recovery. All walks were at 4.0 mph.
He 5 rounds at 7.0mph, 7.2mph, 7.3mph, 7.4mph and 7.5mph for 1 minute each with 1 minute recovery at 4.0mph. Then we tried the last round for 2 minutes at 7.0mph which he only did for about 90 seconds as he was spent. He spent the rest of his time walking at 4.0mph.
I told him to stretch really good afterwards as running makes your hamstrings tight, which will make your glutes tight, which will eventually find its way to your back. He sits for a living so that is a recipe for lower-back pain.
He thanked me for my coaching and admitted he would never have challenged himself the way I did.
3 times a week should get the job done. After you get fitter, come back and we can tweak this recipe. Hope this helps.