There are lots of benefits to even just a small amount of exercise a day, including reducing your risk from long term conditions.
A great way to start is the "Couch to 5K" program.
It takes you from walk to running 5km (3miles) in 9 weeks. You can download the app or print off the instructions. Week one starts with mainly walking and a minute running. The schedule then builds up to eventually run for 30 minutes.
That might seem a long way off but because you gradually work towards that goal, you'll be surprised you can do it!