Ditto Nobby on the 20 minute jog for warmups.
When I stated the above about what we did, I had a good 2 years or more consistent running behind me.
People starting out or in their first year need to adjust accordingly.
I have tried to dig out one of my diaries from "way back" when I started out but seemed to have 'buried' them somewhere.
I am not sure of exactly what we did but I know we jogged nothing less than a mile and did no more than 4 to 6 strides and would have jogged same as we warmed up, if not a little more.
As our overall fitness increased so did the 'volume' (if that is the right word).
Once again : You must work out what suits you.