Running targets during weight loss

Hi all,

To start with some background, in August 2024 when I (M, 36) decided I needed to sort my health out, I was 23 stone (I'm only 5 ft 9). I could barely get up the stairs without needing to catch my breath. We have a treadmill in the garage that was gathering dust and while I couldn't use it at first because it was too painful, as the weight started to come off, I tried running.

At first I was doing 30 seconds running to a minute of walking and 10 minutes of that was killing me. I've been using the treadmill for about 12 months and now run for 25 minutes. 1 minute running to 1 minute walking (with a couple of double minutes thrown in at minute 9 and 18). I'm currently just under 16 stone, so still in an obese BMI but I've been enjoying the running and recently started to go for a morning run (outside, where other people are).

Having targets that I can measure and achieve has been really helpful for me and I feel like a really big target is to run a marathon when I'm 40. I turn 37 this summer, I'm still nearly four stone above a healthy BMI, so these would be two targets I'm aiming towards simultaneously and I think they're pretty complimentary. However I've never so much as ran a 5k. My morning run is around 3k and I still need to slow to a walk multiple times. I've never been fit in my entire life so I have no idea what kind of progression I should expect if I maintain consistency or if the idea of getting fit enough to run a marathon in three years time from my current fitness level is fantasy.

I was hoping there may be people who have been through building fitness back up like this who could give me advice on targets. Should I be aiming to do my 3k without stopping, then go 5k, 10k etc. Is there an established wisdom on this sort of thing?