Started last year at 19st 10 and cycled 2 short of 3,000 miles. Start this year at 13st 3 with a target of 12.5st and 5,000 miles. Only my seemingly neverending health problems will stop me hitting the mileage target
Plenty of people walk or run/walk ours in Cambridge and work on running more/walking less as the weeks go by. There was a lady in her 80s went round on her zimmer frame time last year. If you can already run 4k in 30 minutes on a treadmill, there will definitely be people slower than you at parkun. Those "couch to 5k" plans the NHS does build you up to running 4k without a break over the space of a couple of months, BTW.
Have you ever seen a doctor/physio about your ankle? It's fairly common for people who start running to get injuries - your cardiovascular system can improve a lot quicker than your joints/tendons/muscles can get stronger, so it's easy to overdo it. Pre-existing injury or not, if one ankle hurts and the other doesn't when out running, I'd probably take that as a sign to take it a bit easier.
That's why cycling is a good option. Low impact exercise on knees and ankles.
Im shopping for a new road bike in the spring and I'll be buying a triple. There is a lot of snobbery and brand loyalty within road cycling but while those riding double sets will be puffing on the steeper climbs, a triple will give you plenty of range to keep spinning the pedals at a useful pace despite your actual speed