As long as you have comfortable shoes, walking is a good budget option. Make it an adventure, set yourself targets like photographing a new wild plant every day, or walking all the streets in your town, or visiting every church. Download Strava onto your phone and use it to log your mileage.
You can enter an online challenge such as the ones the "end to end" people run (pay a bit, log your miles, get a medal when you've done them) if you can spare the cash and want a bit of motivation.
Once you can walk briskly for an hour, start looking for local parkruns, get a sports bra and comfy trainers, and start to walk/jog a local parkrun where people finish in 45 minutes or more. Do the c25k program in the week.
Eat lots of veg and protein and don't buy ice-cream, cake or biscuits, and limit how much bread you buy.
Come back in a year and tell us how you did (or if you hit problems, come back here and ask, someone's bound to have gone through the same stuff).