I did a 10k, and I began by running on the treadmill, slowly increasing my distance to help build up my stamina, and then did some road running to get used to that.
Treadmill running is very different to roads, so I would suggest doing plenty of road runs to get used to it
I've heard very good things about the Couch to 5k app though. I didn't use it myself, but I've got friends in my SW group that never exercised at all, and they used that app to get themselves up and moving. One started using the app about 12 months ago and has recently done a half marathon! She's going for the London marathon next year
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